diff --git a/cmd/thyme/main.go b/cmd/thyme/main.go
index 9ea4c9d..12476bc 100644
--- a/cmd/thyme/main.go
+++ b/cmd/thyme/main.go
@@ -6,6 +6,7 @@ import (
 	"log"
 	"os"
 	"runtime"
+	"time"
 
 	"github.com/jessevdk/go-flags"
 	"github.com/sourcegraph/thyme"
@@ -17,14 +18,44 @@ func init() {
 	if _, err := CLI.AddCommand("track", "", "record current windows", &trackCmd); err != nil {
 		log.Fatal(err)
 	}
-	if _, err := CLI.AddCommand("show", "", "visualize data", &showCmd); err != nil {
+	if _, err := CLI.AddCommand("watch", "", "record current windows at regular intervals (default 30s)", &watchCmd); err != nil {
 		log.Fatal(err)
 	}
-	if _, err := CLI.AddCommand("dep", "", "external dependencies that need to be installed", &depCmd); err != nil {
+	if _, err := CLI.AddCommand("show", "", "visualize data", &showCmd); err != nil {
 		log.Fatal(err)
 	}
 }
 
+// WatchCmd is the subcommand that tracks application usage at regular intervals.
+type WatchCmd struct {
+	// The track command is a subset of the watch command
+	TrackCmd
+	Interval int64 `long:"interval" short:"n" description:"update interval (default 30 seconds)"`
+}
+
+var watchCmd WatchCmd
+
+func (c *WatchCmd) Execute(args []string) error {
+	var interval time.Duration
+	if c.Interval <= 0 {
+		// Set default interval
+		interval = 30 * time.Second
+	} else {
+		interval = time.Duration(c.Interval) * time.Second
+	}
+
+	// Loop until the user aborts the command
+	for {
+		err := c.TrackCmd.Execute(args)
+		if err != nil {
+			return err
+		}
+
+		// Sleep for a while until the next time we should track active windows
+		time.Sleep(interval)
+	}
+}
+
 // TrackCmd is the subcommand that tracks application usage.
 type TrackCmd struct {
 	Out string `long:"out" short:"o" description:"output file"`
@@ -123,19 +154,6 @@ func (c *ShowCmd) Execute(args []string) error {
 	return nil
 }
 
-type DepCmd struct{}
-
-var depCmd DepCmd
-
-func (c *DepCmd) Execute(args []string) error {
-	t, err := getTracker()
-	if err != nil {
-		return err
-	}
-	fmt.Println(t.Deps())
-	return nil
-}
-
 func main() {
 	run := func() error {
 		_, err := CLI.Parse()
diff --git a/darwin.go b/darwin.go
index 8ae0612..7d769c7 100644
--- a/darwin.go
+++ b/darwin.go
@@ -90,13 +90,11 @@ end repeat
 `
 )
 
-func (t *DarwinTracker) Deps() string {
-	return `
-You will need the osascript command-line utility. You can install it via the Apple developer tools ('xcode-select --install') or npm ('npm install --save osascript').
-
-You will need to enable privileges for "Terminal" in System Preferences > Security & Privacy > Privacy > Accessibility.
-See https://support.apple.com/en-us/HT202802 for details.
-`
+func (t *DarwinTracker) CheckDependencies() {
+	_, err := exec.LookPath("osascript")
+	if err != nil {
+		log.Fatal("You will need the osascript command-line utility. You can install it via the Apple developer tools ('xcode-select --install') or npm ('npm install --save osascript').")
+	}
 }
 
 func (t *DarwinTracker) Snap() (*Snapshot, error) {
@@ -186,7 +184,22 @@ func runAS(script string) (map[process][]*Window, error) {
 	cmd.Stdin = bytes.NewBuffer([]byte(script))
 	b, err := cmd.CombinedOutput()
 	if err != nil {
-		return nil, fmt.Errorf("AppleScript error: %s, output was:\n%s", err, string(b))
+		// This is the error code for 'osascript is not allowed assistive access'.
+		// Add a more informative error message than the one applescript normally outputs.
+		if strings.Contains(string(b), "-25211") {
+			formatString := `
+AppleScript error: %s
+You will need to enable privileges for "Terminal" (or Iterm2 if you are using that) in
+System Preferences > Security & Privacy > Privacy > Accessibility.
+See https://support.apple.com/en-us/HT202802 for details.
+
+output was:
+%s`
+
+			return nil, fmt.Errorf(formatString, err, string(b))
+		} else {
+			return nil, fmt.Errorf("AppleScript error: %s, output was:\n%s", err, string(b))
+		}
 	}
 	return parseASOutput(string(b))
 }
diff --git a/data.go b/data.go
index 66889e7..2f0f03c 100644
--- a/data.go
+++ b/data.go
@@ -25,7 +25,9 @@ func NewTracker(name string) Tracker {
 	if _, exists := trackers[name]; !exists {
 		log.Fatalf("no Tracker constructor has been registered with name %s", name)
 	}
-	return trackers[name]()
+	tracker := trackers[name]()
+	tracker.CheckDependencies()
+	return tracker
 }
 
 // Tracker tracks application usage. An implementation that satisfies
@@ -36,9 +38,10 @@ type Tracker interface {
 	// at the current time.
 	Snap() (*Snapshot, error)
 
-	// Deps returns a string listing the dependencies that still need
-	// to be installed with instructions for how to install them.
-	Deps() string
+	// CheckDependencies checks for external dependencies (for example
+	// 'osascript' on OS X) and logs a fatal error if they are not available
+	// as well as instructions for how to install them.
+	CheckDependencies()
 }
 
 // Stream represents all the sampling data gathered by Thyme.
diff --git a/linux.go b/linux.go
index 4fd2210..f8d9618 100644
--- a/linux.go
+++ b/linux.go
@@ -2,6 +2,7 @@ package thyme
 
 import (
 	"fmt"
+	"os"
 	"os/exec"
 	"regexp"
 	"strconv"
@@ -22,13 +23,26 @@ func NewLinuxTracker() Tracker {
 	return &LinuxTracker{}
 }
 
-func (t *LinuxTracker) Deps() string {
-	return `Install the following command-line utilities via your package manager (e.g., apt) of choice:
-* xdpyinfo
-* xwininfo
-* xdotool
-* wmctrl
-`
+func (t *LinuxTracker) CheckDependencies() {
+	deps := map[string]string{
+		"xdpyinfo": "x11-utils",
+		"xwininfo": "x11-utils",
+		"xdotool":  "xdotool",
+		"wmctrl":   "wmctrl",
+	}
+
+	anyFailed := false
+	for k, v := range deps {
+		_, err := exec.LookPath(k)
+		if err != nil {
+			fmt.Printf("You need to install the command line utility '%s' (usually in the package named '%s') via your package manager of choice.\nFor example 'apt-get install %s'\n\n", k, v, v)
+			anyFailed = true
+		}
+	}
+
+	if anyFailed {
+		os.Exit(1)
+	}
 }
 
 func (t *LinuxTracker) Snap() (*Snapshot, error) {
diff --git a/windows.go b/windows.go
index 3aeb167..927309b 100644
--- a/windows.go
+++ b/windows.go
@@ -40,8 +40,8 @@ var (
 	procGetWindowThreadProcessId = user.NewProc("GetWindowThreadProcessId")
 )
 
-func (t *WindowsTracker) Deps() string {
-	return "Nothing, Ready to Go!"
+func (t *WindowsTracker) CheckDependencies() {
+	// Nothing, Ready to Go!
 }
 
 // getWindowTitle returns a title of a window of the provided system window handle