Bayesian Blocks is a new time series algorithm for detecting localized structures (spikesorshots), revealing pulse shapes, and generally characterizing intensity variations. It maps raw counting data into a maximum likelihood piecewise constant representation of the underlying signal. This bin-free method imposes no lower limit on measurable time scales. Applied to BATSE TTE data, it reveals the shortest known burst structure—a spike superimposed on the main burst in GRB920229 (BATSE trigger 1453), with rise and decaytimescales≈few100 &mgr;s. ©1998 American Institute of Physics.