Measurements of trajectories of individual proteins or lipids in the p
lasma membrane of cells show a variety of types of motion. Brownian mo
tion is observed, but many of the particles undergo non-Brownian motio
n, including directed motion, confined motion, and anomalous diffusion
. The variety of motion leads to significant effects on the kinetics o
f reactions among membrane-bound species and requires a revision of ex
isting views of membrane structure and dynamics.