macOS Sierra (10.12) or later

Mac: System Preferences

Since macOS Sierra Version 10.12.1, it is easy to map Caps Lock to Escape via System Preferences. Perform the following steps to do so.

Go to the Apple menu > System Preferences > Keyboard.
Click Modifier Keys.
Set Caps Lock Key to Escape.
Click OK.



Older OS X versions:
Mac: Seil

In OS X, the mapping options available via System Preferences are very limited. For example, it does not allow Caps Lock to be mapped to Escape. Therefore in general, it becomes necessary to install additional software to map keys in OS X.

A popular tool for mapping one key to another on OS X is Seil. Here is an article by my beautiful girlfriend and wife that describes how to map Caps Lock to Escape on OS X using Seil: http://sunainapai.in/blog/map-caps-lock-to-escape-on-mac-os-x/.