Teachers may engage in end-user programming to support student learning or administrative activities associated with teaching. The objective of this research is to understand strategies used by teachers in program comprehension and to identify specific problems they face. A think-aloud study was conducted of teachers comprehending an event-driven application, consisting of a graphical user interface and the scripts controlling it. We found that end users followed a strongly top-down strategy and breadth-wise exploration of the application. Depth-wise exploration was observed in half the teachers. Teachers varied greatly in their motivations and persistence to dig deeply into the code. Problems of the teachers included difficulties comprehending the event-driven application, given the distributed nature of the code, choosing appropriate inputs for running the program, and reasoning about the results of their test runs.