The circadian clock evolved under entraining conditions, yet most circadian experiments and much circadian theory are built around free-running rhythms. The interpretation of entrainment experiments is certainly more complex than that of free-running rhythms due to the relationship between exogenous and endogenous cycles. Here, we systematically describe entrainment in the simplest of the traditional eukaryotic model systems in circadian research, Neurospora crassa. This fungus forms a mass of spores (bands of conidia) each day. Over a wide range of photoperiods, these bands begin to appear at midnight, suggesting integration of neither dawn nor dusk signals alone. However, when symmetrical light/dark cycles (T cycles, each with 50% light) are applied, dusk determines the time of conidiation with a uniform, period-dependent delay in phase. This "forced" synchronization appears to be specific for the zeitgeber light because similar experiments, but using temperature, result in systematic entrainment, with bands appearing relatively later in shorter cycles and earlier in longer cycles. We find that the molecular mechanism of entrainment primarily concerns posttranscriptional regulation. Finally, we have used Neurospora to investigate acute effects of zeitgeber stimuli known as "masking."