A common belief is that software designs decay as systems evolve. This research examines the extent to which software designs actually decay by studying the aging of design patterns in successful object oriented systems. Aging of design patterns is measured using various types of decay indices developed for this research. Decay indices track the internal structural changes of a design pattern realization and the code that surrounds the realization. Hypotheses for each kind of decay are tested. We found that the original design pattern functionality remains, and pattern decay is due to the "grime", non-pattern code, that grows around the pattern realization.