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We use our own, recently developed preÈmain-sequence evolutionary tracks to investigate the star formation histories of relatively nearby associations and clusters. We Ðrst employ published luminosities and e †ective temperatures to place the known members of each region in the H-R diagram. We then construct age histograms detailing that regionÏs history. The groups studied include Taurus-Auriga, Lupus, Chamaeleon, o Ophiuchi, Upper Scorpius, IC 348, and NGC 2264. This study is the Ðrst to analyze a large number of star-forming regions with the same set of theoretical tracks.Our investigation corroborates and extends our previous results on the Orion Nebula Cluster. In all cases, we Ðnd that star formation began at a relatively low level some 107 yr in the past and has more recently undergone a steep acceleration. This acceleration, which lasts several million years, is usually continuing through the present epoch. The one clear exception is the OB association Upper Scorpius, where the formation rate climbed upward, peaked, and has now died o †. SigniÐcantly, this is also the only region of our list that has been largely stripped of molecular gas.The acceleration represents a true physical phenomenon that cannot be explained away by incompleteness of the samples ; nor is the pattern of stellar births signiÐcantly a †ected by observational errors or the presence of unresolved binaries. We speculate that increasing star formation activity arises from contraction of the parent cloud. Despite the short timescale for acceleration, the cloud is likely to evolve quasi-statically. Star formation itself appears to be a critical phenomenon, occurring only in locations exceeding some threshold density. The cloudÏs contraction must reverse itself, and the remnant gas dissipate, in less than 107 yr, even for aggregates containing no massive stars. In this case, molecular outÑows from the stars themselves presumably accomplish the task, but the actual dispersal mechanism is still unclear.