diff --git a/webdav/if.go b/webdav/if.go
new file mode 100644
index 00000000..e4d46706
--- /dev/null
+++ b/webdav/if.go
@@ -0,0 +1,181 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package webdav
+
+// The If header is covered by Section 10.4.
+// http://www.webdav.org/specs/rfc4918.html#HEADER_If
+
+import (
+	"strings"
+)
+
+// ifHeader is a disjunction (OR) of ifLists.
+type ifHeader struct {
+	lists []ifList
+}
+
+// ifList is a conjunction (AND) of ifConditions, and an optional resource tag.
+type ifList struct {
+	resourceTag string
+	conditions  []ifCondition
+}
+
+// ifCondition can match a WebDAV resource, based on a stateToken or ETag.
+// Exactly one of stateToken and entityTag should be non-empty.
+type ifCondition struct {
+	not        bool
+	stateToken string
+	entityTag  string
+}
+
+// parseIfHeader parses the "If: foo bar" HTTP header. The httpHeader string
+// should omit the "If:" prefix and have any "\r\n"s collapsed to a " ", as is
+// returned by req.Header.Get("If") for a http.Request req.
+func parseIfHeader(httpHeader string) (h ifHeader, ok bool) {
+	s := strings.TrimSpace(httpHeader)
+	switch tokenType, _, _ := lex(s); tokenType {
+	case '(':
+		return parseNoTagLists(s)
+	case angleTokenType:
+		return parseTaggedLists(s)
+	default:
+		return ifHeader{}, false
+	}
+}
+
+func parseNoTagLists(s string) (h ifHeader, ok bool) {
+	for {
+		l, remaining, ok := parseList(s)
+		if !ok {
+			return ifHeader{}, false
+		}
+		h.lists = append(h.lists, l)
+		if remaining == "" {
+			return h, true
+		}
+		s = remaining
+	}
+}
+
+func parseTaggedLists(s string) (h ifHeader, ok bool) {
+	resourceTag, n := "", 0
+	for first := true; ; first = false {
+		tokenType, tokenStr, remaining := lex(s)
+		switch tokenType {
+		case angleTokenType:
+			if !first && n == 0 {
+				return ifHeader{}, false
+			}
+			resourceTag, n = tokenStr, 0
+			s = remaining
+		case '(':
+			n++
+			l, remaining, ok := parseList(s)
+			if !ok {
+				return ifHeader{}, false
+			}
+			l.resourceTag = resourceTag
+			h.lists = append(h.lists, l)
+			if remaining == "" {
+				return h, true
+			}
+			s = remaining
+		default:
+			return ifHeader{}, false
+		}
+	}
+}
+
+func parseList(s string) (l ifList, remaining string, ok bool) {
+	tokenType, _, s := lex(s)
+	if tokenType != '(' {
+		return ifList{}, "", false
+	}
+	for {
+		tokenType, _, remaining = lex(s)
+		if tokenType == ')' {
+			if len(l.conditions) == 0 {
+				return ifList{}, "", false
+			}
+			return l, remaining, true
+		}
+		c, remaining, ok := parseCondition(s)
+		if !ok {
+			return ifList{}, "", false
+		}
+		l.conditions = append(l.conditions, c)
+		s = remaining
+	}
+}
+
+func parseCondition(s string) (c ifCondition, remaining string, ok bool) {
+	tokenType, tokenStr, s := lex(s)
+	if tokenType == notTokenType {
+		c.not = true
+		tokenType, tokenStr, s = lex(s)
+	}
+	switch tokenType {
+	case strTokenType, angleTokenType:
+		c.stateToken = tokenStr
+	case squareTokenType:
+		c.entityTag = tokenStr
+	default:
+		return ifCondition{}, "", false
+	}
+	return c, s, true
+}
+
+// Single-rune tokens like '(' or ')' have a token type equal to their rune.
+// All other tokens have a negative token type.
+const (
+	errTokenType    = rune(-1)
+	eofTokenType    = rune(-2)
+	strTokenType    = rune(-3)
+	notTokenType    = rune(-4)
+	angleTokenType  = rune(-5)
+	squareTokenType = rune(-6)
+)
+
+func lex(s string) (tokenType rune, tokenStr string, remaining string) {
+	// The net/textproto Reader that parses the HTTP header will collapse
+	// Linear White Space that spans multiple "\r\n" lines to a single " ",
+	// so we don't need to look for '\r' or '\n'.
+	for len(s) > 0 && (s[0] == '\t' || s[0] == ' ') {
+		s = s[1:]
+	}
+	if len(s) == 0 {
+		return eofTokenType, "", ""
+	}
+	i := 0
+loop:
+	for ; i < len(s); i++ {
+		switch s[i] {
+		case '\t', ' ', '(', ')', '<', '>', '[', ']':
+			break loop
+		}
+	}
+
+	if i != 0 {
+		tokenStr, remaining = s[:i], s[i:]
+		if tokenStr == "Not" {
+			return notTokenType, "", remaining
+		}
+		return strTokenType, tokenStr, remaining
+	}
+
+	j := 0
+	switch s[0] {
+	case '<':
+		j, tokenType = strings.IndexByte(s, '>'), angleTokenType
+	case '[':
+		j, tokenType = strings.IndexByte(s, ']'), squareTokenType
+	default:
+		return rune(s[0]), "", s[1:]
+	}
+	if j < 0 {
+		return errTokenType, "", ""
+	}
+	return tokenType, s[1:j], s[j+1:]
+}
diff --git a/webdav/if_test.go b/webdav/if_test.go
new file mode 100644
index 00000000..684e8b32
--- /dev/null
+++ b/webdav/if_test.go
@@ -0,0 +1,322 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package webdav
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+)
+
+func TestParseIfHeader(t *testing.T) {
+	// The "section x.y.z" test cases come from section x.y.z of the spec at
+	// http://www.webdav.org/specs/rfc4918.html
+	testCases := []struct {
+		desc  string
+		input string
+		want  ifHeader
+	}{{
+		"bad: empty",
+		``,
+		ifHeader{},
+	}, {
+		"bad: no parens",
+		`foobar`,
+		ifHeader{},
+	}, {
+		"bad: empty list #1",
+		`()`,
+		ifHeader{},
+	}, {
+		"bad: empty list #2",
+		`(a) (b c) () (d)`,
+		ifHeader{},
+	}, {
+		"bad: no list after resource #1",
+		`<foo>`,
+		ifHeader{},
+	}, {
+		"bad: no list after resource #1",
+		`<foo> <bar> (a)`,
+		ifHeader{},
+	}, {
+		"bad: no list after resource #3",
+		`<foo> (a) (b) <bar>`,
+		ifHeader{},
+	}, {
+		"bad: no-tag-list followed by tagged-list",
+		`(a) (b) <foo> (c)`,
+		ifHeader{},
+	}, {
+		"bad: unfinished list",
+		`(a`,
+		ifHeader{},
+	}, {
+		"bad: unfinished ETag",
+		`([b`,
+		ifHeader{},
+	}, {
+		"bad: unfinished Notted list",
+		`(Not a`,
+		ifHeader{},
+	}, {
+		"bad: double Not",
+		`(Not Not a)`,
+		ifHeader{},
+	}, {
+		"good: one list with a stateToken",
+		`(a)`,
+		ifHeader{
+			lists: []ifList{{
+				conditions: []ifCondition{{
+					stateToken: `a`,
+				}},
+			}},
+		},
+	}, {
+		"good: one list with an ETag",
+		`([a])`,
+		ifHeader{
+			lists: []ifList{{
+				conditions: []ifCondition{{
+					entityTag: `a`,
+				}},
+			}},
+		},
+	}, {
+		"good: one list with three Nots",
+		`(Not a Not b Not [d])`,
+		ifHeader{
+			lists: []ifList{{
+				conditions: []ifCondition{{
+					not:        true,
+					stateToken: `a`,
+				}, {
+					not:        true,
+					stateToken: `b`,
+				}, {
+					not:       true,
+					entityTag: `d`,
+				}},
+			}},
+		},
+	}, {
+		"good: two lists",
+		`(a) (b)`,
+		ifHeader{
+			lists: []ifList{{
+				conditions: []ifCondition{{
+					stateToken: `a`,
+				}},
+			}, {
+				conditions: []ifCondition{{
+					stateToken: `b`,
+				}},
+			}},
+		},
+	}, {
+		"good: two Notted lists",
+		`(Not a) (Not b)`,
+		ifHeader{
+			lists: []ifList{{
+				conditions: []ifCondition{{
+					not:        true,
+					stateToken: `a`,
+				}},
+			}, {
+				conditions: []ifCondition{{
+					not:        true,
+					stateToken: `b`,
+				}},
+			}},
+		},
+	}, {
+		"section 7.5.1",
+		`<http://www.example.com/users/f/fielding/index.html> 
+			(<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>)`,
+		ifHeader{
+			lists: []ifList{{
+				resourceTag: `http://www.example.com/users/f/fielding/index.html`,
+				conditions: []ifCondition{{
+					stateToken: `urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6`,
+				}},
+			}},
+		},
+	}, {
+		"section 7.5.2 #1",
+		`(<urn:uuid:150852e2-3847-42d5-8cbe-0f4f296f26cf>)`,
+		ifHeader{
+			lists: []ifList{{
+				conditions: []ifCondition{{
+					stateToken: `urn:uuid:150852e2-3847-42d5-8cbe-0f4f296f26cf`,
+				}},
+			}},
+		},
+	}, {
+		"section 7.5.2 #2",
+		`<http://example.com/locked/>
+			(<urn:uuid:150852e2-3847-42d5-8cbe-0f4f296f26cf>)`,
+		ifHeader{
+			lists: []ifList{{
+				resourceTag: `http://example.com/locked/`,
+				conditions: []ifCondition{{
+					stateToken: `urn:uuid:150852e2-3847-42d5-8cbe-0f4f296f26cf`,
+				}},
+			}},
+		},
+	}, {
+		"section 7.5.2 #3",
+		`<http://example.com/locked/member>
+			(<urn:uuid:150852e2-3847-42d5-8cbe-0f4f296f26cf>)`,
+		ifHeader{
+			lists: []ifList{{
+				resourceTag: `http://example.com/locked/member`,
+				conditions: []ifCondition{{
+					stateToken: `urn:uuid:150852e2-3847-42d5-8cbe-0f4f296f26cf`,
+				}},
+			}},
+		},
+	}, {
+		"section 9.9.6",
+		`(<urn:uuid:fe184f2e-6eec-41d0-c765-01adc56e6bb4>) 
+			(<urn:uuid:e454f3f3-acdc-452a-56c7-00a5c91e4b77>)`,
+		ifHeader{
+			lists: []ifList{{
+				conditions: []ifCondition{{
+					stateToken: `urn:uuid:fe184f2e-6eec-41d0-c765-01adc56e6bb4`,
+				}},
+			}, {
+				conditions: []ifCondition{{
+					stateToken: `urn:uuid:e454f3f3-acdc-452a-56c7-00a5c91e4b77`,
+				}},
+			}},
+		},
+	}, {
+		"section 9.10.8",
+		`(<urn:uuid:e71d4fae-5dec-22d6-fea5-00a0c91e6be4>)`,
+		ifHeader{
+			lists: []ifList{{
+				conditions: []ifCondition{{
+					stateToken: `urn:uuid:e71d4fae-5dec-22d6-fea5-00a0c91e6be4`,
+				}},
+			}},
+		},
+	}, {
+		"section 10.4.6",
+		`(<urn:uuid:181d4fae-7d8c-11d0-a765-00a0c91e6bf2> 
+			["I am an ETag"])
+			(["I am another ETag"])`,
+		ifHeader{
+			lists: []ifList{{
+				conditions: []ifCondition{{
+					stateToken: `urn:uuid:181d4fae-7d8c-11d0-a765-00a0c91e6bf2`,
+				}, {
+					entityTag: `"I am an ETag"`,
+				}},
+			}, {
+				conditions: []ifCondition{{
+					entityTag: `"I am another ETag"`,
+				}},
+			}},
+		},
+	}, {
+		"section 10.4.7",
+		`(Not <urn:uuid:181d4fae-7d8c-11d0-a765-00a0c91e6bf2> 
+			<urn:uuid:58f202ac-22cf-11d1-b12d-002035b29092>)`,
+		ifHeader{
+			lists: []ifList{{
+				conditions: []ifCondition{{
+					not:        true,
+					stateToken: `urn:uuid:181d4fae-7d8c-11d0-a765-00a0c91e6bf2`,
+				}, {
+					stateToken: `urn:uuid:58f202ac-22cf-11d1-b12d-002035b29092`,
+				}},
+			}},
+		},
+	}, {
+		"section 10.4.8",
+		`(<urn:uuid:181d4fae-7d8c-11d0-a765-00a0c91e6bf2>) 
+			(Not <DAV:no-lock>)`,
+		ifHeader{
+			lists: []ifList{{
+				conditions: []ifCondition{{
+					stateToken: `urn:uuid:181d4fae-7d8c-11d0-a765-00a0c91e6bf2`,
+				}},
+			}, {
+				conditions: []ifCondition{{
+					not:        true,
+					stateToken: `DAV:no-lock`,
+				}},
+			}},
+		},
+	}, {
+		"section 10.4.9",
+		`</resource1> 
+			(<urn:uuid:181d4fae-7d8c-11d0-a765-00a0c91e6bf2> 
+			[W/"A weak ETag"]) (["strong ETag"])`,
+		ifHeader{
+			lists: []ifList{{
+				resourceTag: `/resource1`,
+				conditions: []ifCondition{{
+					stateToken: `urn:uuid:181d4fae-7d8c-11d0-a765-00a0c91e6bf2`,
+				}, {
+					entityTag: `W/"A weak ETag"`,
+				}},
+			}, {
+				resourceTag: `/resource1`,
+				conditions: []ifCondition{{
+					entityTag: `"strong ETag"`,
+				}},
+			}},
+		},
+	}, {
+		"section 10.4.10",
+		`<http://www.example.com/specs/> 
+			(<urn:uuid:181d4fae-7d8c-11d0-a765-00a0c91e6bf2>)`,
+		ifHeader{
+			lists: []ifList{{
+				resourceTag: `http://www.example.com/specs/`,
+				conditions: []ifCondition{{
+					stateToken: `urn:uuid:181d4fae-7d8c-11d0-a765-00a0c91e6bf2`,
+				}},
+			}},
+		},
+	}, {
+		"section 10.4.11 #1",
+		`</specs/rfc2518.doc> (["4217"])`,
+		ifHeader{
+			lists: []ifList{{
+				resourceTag: `/specs/rfc2518.doc`,
+				conditions: []ifCondition{{
+					entityTag: `"4217"`,
+				}},
+			}},
+		},
+	}, {
+		"section 10.4.11 #2",
+		`</specs/rfc2518.doc> (Not ["4217"])`,
+		ifHeader{
+			lists: []ifList{{
+				resourceTag: `/specs/rfc2518.doc`,
+				conditions: []ifCondition{{
+					not:       true,
+					entityTag: `"4217"`,
+				}},
+			}},
+		},
+	}}
+
+	for _, tc := range testCases {
+		got, ok := parseIfHeader(strings.Replace(tc.input, "\n", "", -1))
+		if gotEmpty := reflect.DeepEqual(got, ifHeader{}); gotEmpty == ok {
+			t.Errorf("%s: should be different: empty header == %t, ok == %t", tc.desc, gotEmpty, ok)
+			continue
+		}
+		if !reflect.DeepEqual(got, tc.want) {
+			t.Errorf("%s:\ngot  %v\nwant %v", tc.desc, got, tc.want)
+			continue
+		}
+	}
+}