  We review the observations of rapid flux density variations in compact radio
sources, and discuss the inverse Compton limit to the maximum brightness
temperature of incoherent synchrotron sources in comparison with recent VLBA
observations. The apparent agreement of the theoretical brightness temperature
limit due to inverse Compton cooling and the brightness temperatures observed
by early VLBI observations appears to have been fortuitous. VLBA observations
have greatly improved the quality of the data, but many of the early issues
remain unresolved.
