  Many crucial biological processes operate with surprisingly small numbers of
molecules, and there is renewed interest in analyzing the impact of noise
associated with these small numbers. Twenty--five years ago, Berg and Purcell
showed that bacterial chemotaxis, where a single celled organism must respond
to small changes in concentration of chemicals outside the cell, is limited
directly by molecule counting noise, and that aspects of the bacteria's
behavioral and computational strategies must be chosen to minimize the effects
of this noise. Here we revisit and generalize their arguments to estimate the
physical limits to signaling processes within the cell, and argue that recent
experiments are consistent with performance approaching these limits.
