The data in Section 3 indicates that system failures are predominantly transient in nature and follow a decreasing failure rate function (i.e., Weibull) rather than a constant failure rate function (i.e., exponential). System failures have diverse manifestations and diverse causes ranging from errors in design to component aging. A substantial gap remains between actual system failures and these system failure models. Perhaps as little as 50of the observed system faiures fall into one or more of the high-level fault models defined in Section 1. Much work remains to effectively bridge this gap.