diff --git a/README.md b/README.md
index 4cd8f5d..9fa0332 100644
--- a/README.md
+++ b/README.md
@@ -142,6 +142,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 
 - [All The Environment Variables](zsh/all-the-environment-variables.md)
 - [Cat A File With Line Numbers](zsh/cat-a-file-with-line-numbers.md)
+- [Check If A Port Is In Use](zsh/check-if-a-port-is-in-use.md)
 - [Clear The Screen](zsh/clear-the-screen.md)
 - [Create A File Descriptor with Process Substitution](zsh/create-a-file-descriptor-with-process-substitution.md)
 - [Killing A Frozen SSH Session](zsh/killing-a-frozen-ssh-session.md)
diff --git a/zsh/check-if-a-port-is-in-use.md b/zsh/check-if-a-port-is-in-use.md
new file mode 100644
index 0000000..fbc4e05
--- /dev/null
+++ b/zsh/check-if-a-port-is-in-use.md
@@ -0,0 +1,18 @@
+# Check If A Port Is In Use
+
+The `lsof` command is used to *list open files*. This includes listing
+network connections. This means I can check if a particular port is in use
+and what process is using that port. For instance, I can check if my rails
+application is currently running on port 3000.
+
+```
+$ lsof -i TCP:3000
+COMMAND   PID       USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
+ruby    13821 jbranchaud   12u  IPv6 0xdf2e9fd346cc12b5      0t0  TCP localhost:hbci (LISTEN)
+ruby    13821 jbranchaud   13u  IPv4 0xdf2e9fd33ca74d65      0t0  TCP localhost:hbci (LISTEN)
+```
+
+I can see that a ruby process (my rails app) is using port 3000. The PID
+and a number of other details are included.
+
+See more details with `man lsof`.