On age spreads in star-forming regions

I consider current observational constraints on age spreads in star-forming regions and recent attempts to infer the star-forming histories of molecular cloud complexes. My analysis concentrates on low-mass pre-main-sequence stars on Hayashi tracks, which constitute the bulk of the stellar population. Current observational and theoretical uncertainties probably preclude any robust inferences of the detailed variation of star formation rates as a function of time. These results do not change the conclusion, supported again recently by the systematic study of Palla & Stahler, that star formation is rapid.