diff --git a/cli/all.go b/cli/all.go
index a248e9a..50e356f 100644
--- a/cli/all.go
+++ b/cli/all.go
@@ -26,11 +26,11 @@ type Termshark struct {
 	CaptureFilter   string         `short:"f" description:"Apply capture filter." value-name:"<capture filter>"`
 	TimestampFormat string         `short:"t" description:"Set the format of the packet timestamp printed in summary lines." choice:"a" choice:"ad" choice:"adoy" choice:"d" choice:"dd" choice:"e" choice:"r" choice:"u" choice:"ud" choice:"udoy" value-name:"<timestamp format>"`
 	PlatformSwitches
-	PassThru string `long:"pass-thru" default:"auto" optional:"true" optional-value:"true" choice:"auto" choice:"true" choice:"false" description:"Run tshark instead (auto => if stdout is not a tty)."`
-	LogTty   bool   `long:"log-tty" optional:"true" optional-value:"true" choice:"true" choice:"false" description:"Log to the terminal."`
-	Debug    string `long:"debug" default:"false" hidden:"true" optional:"true" optional-value:"true" choice:"true" choice:"false" description:"Enable termshark debugging. See https://termshark.io/userguide."`
-	Help     bool   `long:"help" short:"h" optional:"true" optional-value:"true" description:"Show this help message."`
-	Version  []bool `long:"version" short:"v" optional:"true" optional-value:"true" description:"Show version information."`
+	PassThru string   `long:"pass-thru" default:"auto" optional:"true" optional-value:"true" choice:"auto" choice:"true" choice:"false" description:"Run tshark instead (auto => if stdout is not a tty)."`
+	LogTty   bool     `long:"log-tty" optional:"true" optional-value:"true" choice:"true" choice:"false" description:"Log to the terminal."`
+	Debug    TriState `long:"debug" default:"unset" hidden:"true" optional:"true" optional-value:"true" description:"Enable termshark debugging. See https://termshark.io/userguide."`
+	Help     bool     `long:"help" short:"h" optional:"true" optional-value:"true" description:"Show this help message."`
+	Version  []bool   `long:"version" short:"v" optional:"true" optional-value:"true" description:"Show version information."`
 
 	Args struct {
 		FilterOrPcap string `value-name:"<filter-or-file>" description:"Filter (capture for iface, display for pcap), or pcap to read."`
diff --git a/cli/tristate.go b/cli/tristate.go
new file mode 100644
index 0000000..a4ee789
--- /dev/null
+++ b/cli/tristate.go
@@ -0,0 +1,45 @@
+// Copyright 2019-2020 Graham Clark. All rights reserved.  Use of this source
+// code is governed by the MIT license that can be found in the LICENSE
+// file.
+//
+
+package cli
+
+//======================================================================
+
+type TriState struct {
+	Set bool
+	Val bool
+}
+
+func (b *TriState) UnmarshalFlag(value string) error {
+	switch value {
+	case "true", "TRUE", "t", "T", "1", "y", "Y", "yes", "Yes", "YES":
+		b.Set = true
+		b.Val = true
+	case "false", "FALSE", "f", "F", "0", "n", "N", "no", "No", "NO":
+		b.Set = true
+		b.Val = false
+	default:
+		b.Set = false
+	}
+	return nil
+}
+
+func (b TriState) MarshalFlag() string {
+	if b.Set {
+		if b.Val {
+			return "true"
+		} else {
+			return "false"
+		}
+	} else {
+		return "unset"
+	}
+}
+
+//======================================================================
+// Local Variables:
+// mode: Go
+// fill-column: 78
+// End:
diff --git a/cmd/termshark/termshark.go b/cmd/termshark/termshark.go
index beb589c..f12301d 100644
--- a/cmd/termshark/termshark.go
+++ b/cmd/termshark/termshark.go
@@ -271,6 +271,7 @@ func cmain() int {
 	// terminal emumlator supports 256 colors.
 	termVar := termshark.ConfString("main.term", "")
 	if termVar != "" {
+		log.Infof("Configuration file overrides TERM setting, using TERM=%s", termVar)
 		os.Setenv("TERM", termVar)
 	}
 
@@ -468,7 +469,12 @@ func cmain() int {
 		log.SetOutput(logfd)
 	}
 
-	if cli.FlagIsTrue(opts.Debug) {
+	debug := false
+	if (opts.Debug.Set && opts.Debug.Val == true) || (!opts.Debug.Set && termshark.ConfBool("main.debug", false)) {
+		debug = true
+	}
+
+	if debug {
 		for _, addr := range termshark.LocalIPs() {
 			log.Infof("Starting debug web server at http://%s:6060/debug/pprof/", addr)
 		}
@@ -1134,14 +1140,14 @@ Loop:
 					uiSuspended = false
 				}
 			} else if system.IsSigUSR1(sig) {
-				if cli.FlagIsTrue(opts.Debug) {
+				if debug {
 					termshark.ProfileCPUFor(20)
 				} else {
 					log.Infof("SIGUSR1 ignored by termshark - see the --debug flag")
 				}
 
 			} else if system.IsSigUSR2(sig) {
-				if cli.FlagIsTrue(opts.Debug) {
+				if debug {
 					termshark.ProfileHeap()
 				} else {
 					log.Infof("SIGUSR2 ignored by termshark - see the --debug flag")