Breakpoints

Click to the right of line numbers, or press F9 on a line, to set / remove breakpoints.  Breakpoints are only active when running a script through the Debug command. (Shift + F5).
 
In Debug mode, the active line of execution is indicated with a yellow arrow.  Use the Watches window to view variables / contents.
 
Editing a script whilst debugging will confuse everyone and cause line number mismatches with breakpoints.
 
Keyboard shortcuts when in Breakpoint / debugging paused mode: