diff --git a/public/compose.html b/public/compose.html
index 933083c..e35b0fd 100644
--- a/public/compose.html
+++ b/public/compose.html
@@ -8,11 +8,13 @@
 
 <h2>Compose new message</h2>
 
-<form method="post" action="/compose">
+<form method="post" action="">
+	<input type="hidden" name="in_reply_to" value="{{.Message.InReplyTo}}">
+
 	<p>From:</p>
 	<input type="email" name="from" value="{{.Message.From}}">
 	<p>To:</p>
-	<input type="email" name="to" multiple>
+	<input type="email" name="to" multiple value="{{.Message.ToString}}">
 	<p>Subject:</p>
 	<input type="text" name="subject" value="{{.Message.Subject}}">
 	<p>Body:</p>
diff --git a/public/message.html b/public/message.html
index fc97bf4..2e1a308 100644
--- a/public/message.html
+++ b/public/message.html
@@ -40,6 +40,7 @@
 <hr>
 
 {{if .Body}}
+	<p><a href="{{.Message.Uid}}/reply?part={{.PartPath}}">Reply</a></p>
 	<pre>{{.Body}}</pre>
 {{else}}
 	<p>Can't preview this message part.</p>
diff --git a/server.go b/server.go
index 1de4eae..af84d4f 100644
--- a/server.go
+++ b/server.go
@@ -142,11 +142,7 @@ func handleLogin(ectx echo.Context) error {
 }
 
 func handleGetPart(ctx *context, raw bool) error {
-	mboxName, err := url.PathUnescape(ctx.Param("mbox"))
-	if err != nil {
-		return echo.NewHTTPError(http.StatusBadRequest, err)
-	}
-	uid, err := parseUid(ctx.Param("uid"))
+	mboxName, uid, err := parseMboxAndUid(ctx.Param("mbox"), ctx.Param("uid"))
 	if err != nil {
 		return echo.NewHTTPError(http.StatusBadRequest, err)
 	}
@@ -219,6 +215,61 @@ func handleCompose(ectx echo.Context) error {
 		msg.From = ctx.session.username
 	}
 
+	if ctx.Request().Method == http.MethodGet && ctx.Param("uid") != "" {
+		// This is a reply
+		mboxName, uid, err := parseMboxAndUid(ctx.Param("mbox"), ctx.Param("uid"))
+		if err != nil {
+			return echo.NewHTTPError(http.StatusBadRequest, err)
+		}
+		partPath, err := parsePartPath(ctx.QueryParam("part"))
+		if err != nil {
+			return echo.NewHTTPError(http.StatusBadRequest, err)
+		}
+
+		var inReplyTo *imapMessage
+		var part *message.Entity
+		err = ctx.session.Do(func(c *imapclient.Client) error {
+			var err error
+			inReplyTo, part, err = getMessagePart(c, mboxName, uid, partPath)
+			return err
+		})
+		if err != nil {
+			return err
+		}
+
+		mimeType, _, err := part.Header.ContentType()
+		if err != nil {
+			return fmt.Errorf("failed to parse part Content-Type: %v", err)
+		}
+
+		if !strings.HasPrefix(strings.ToLower(mimeType), "text/") {
+			err := fmt.Errorf("cannot reply to \"%v\" part", mimeType)
+			return echo.NewHTTPError(http.StatusBadRequest, err)
+		}
+
+		msg.Text, err = quote(part.Body)
+		if err != nil {
+			return err
+		}
+
+		msg.InReplyTo = inReplyTo.Envelope.MessageId
+		// TODO: populate From from known user addresses and inReplyTo.Envelope.To
+		replyTo := inReplyTo.Envelope.ReplyTo
+		if len(replyTo) == 0 {
+			replyTo = inReplyTo.Envelope.From
+		}
+		if len(replyTo) > 0 {
+			msg.To = make([]string, len(replyTo))
+			for i, to := range replyTo {
+				msg.To[i] = to.MailboxName + "@" + to.HostName
+			}
+		}
+		msg.Subject = inReplyTo.Envelope.Subject
+		if !strings.HasPrefix(strings.ToLower(msg.Subject), "re:") {
+			msg.Subject = "Re: " + msg.Subject
+		}
+	}
+
 	if ctx.Request().Method == http.MethodPost {
 		// TODO: parse address lists
 		from := ctx.FormValue("from")
@@ -374,6 +425,9 @@ func New(imapURL, smtpURL string) *echo.Echo {
 	e.GET("/compose", handleCompose)
 	e.POST("/compose", handleCompose)
 
+	e.GET("/message/:mbox/:uid/reply", handleCompose)
+	e.POST("/message/:mbox/:uid/reply", handleCompose)
+
 	e.Static("/assets", "public/assets")
 
 	return e
diff --git a/smtp.go b/smtp.go
index 0ec1a92..288f332 100644
--- a/smtp.go
+++ b/smtp.go
@@ -1,14 +1,30 @@
 package koushin
 
 import (
+	"bufio"
 	"fmt"
-	"io"
 	"time"
+	"io"
+	"strings"
 
 	"github.com/emersion/go-message/mail"
 	"github.com/emersion/go-smtp"
 )
 
+func quote(r io.Reader) (string, error) {
+	scanner := bufio.NewScanner(r)
+	var builder strings.Builder
+	for scanner.Scan() {
+		builder.WriteString("> ")
+		builder.Write(scanner.Bytes())
+		builder.WriteString("\n")
+	}
+	if err := scanner.Err(); err != nil {
+		return "", fmt.Errorf("quote: failed to read original message: %s", err)
+	}
+	return builder.String(), nil
+}
+
 func (s *Server) connectSMTP() (*smtp.Client, error) {
 	var c *smtp.Client
 	var err error
@@ -34,10 +50,15 @@ func (s *Server) connectSMTP() (*smtp.Client, error) {
 }
 
 type OutgoingMessage struct {
-	From    string
-	To      []string
+	From string
+	To []string
 	Subject string
-	Text    string
+	InReplyTo string
+	Text string
+}
+
+func (msg *OutgoingMessage) ToString() string {
+	return strings.Join(msg.To, ", ")
 }
 
 func (msg *OutgoingMessage) WriteTo(w io.Writer) error {
@@ -55,6 +76,9 @@ func (msg *OutgoingMessage) WriteTo(w io.Writer) error {
 	if msg.Subject != "" {
 		h.SetText("Subject", msg.Subject)
 	}
+	if msg.InReplyTo != "" {
+		h.Set("In-Reply-To", msg.InReplyTo)
+	}
 
 	mw, err := mail.CreateWriter(w, h)
 	if err != nil {
diff --git a/strconv.go b/strconv.go
index 0879aac..b34241d 100644
--- a/strconv.go
+++ b/strconv.go
@@ -4,12 +4,13 @@ import (
 	"fmt"
 	"strconv"
 	"strings"
+	"net/url"
 )
 
 func parseUid(s string) (uint32, error) {
 	uid, err := strconv.ParseUint(s, 10, 32)
 	if err != nil {
-		return 0, err
+		return 0, fmt.Errorf("invalid UID: %v", err)
 	}
 	if uid == 0 {
 		return 0, fmt.Errorf("UID must be non-zero")
@@ -17,6 +18,15 @@ func parseUid(s string) (uint32, error) {
 	return uint32(uid), nil
 }
 
+func parseMboxAndUid(mboxString, uidString string) (string, uint32, error) {
+	mboxName, err := url.PathUnescape(mboxString)
+	if err != nil {
+		return "", 0, fmt.Errorf("invalid mailbox name: %v", err)
+	}
+	uid, err := parseUid(uidString)
+	return mboxName, uid, err
+}
+
 func parsePartPath(s string) ([]int, error) {
 	if s == "" {
 		return nil, nil