Formal independent testing has been an integral component in the development of EnergyPlus, a new building energy simulation program. Testing to date has included analytical, comparative, sensitivity, range, and empirical tests. Published test suites which include reference results have been applied as much as possible in order to take advantage of the efforts of others to develop well-defined, reproducible tests. The results to date show good agreement with well- established simulation tools such as DOE-2.1E, BLAST, and ESP. Several testing utilities have been developed to help automate the task of assuring that each new version of the software is still performing properly. Selected test results are presented along with lessons learned.