To examine differences in the careers of those involved in terrorist activities over this period, biographical information was examined for 2,512 individuals who were arrested or for whom warrants were issued for committing, planning, or supporting terrorism. Comparison of data for those involved in terrorism before and after 1977 indicates that early terrorists were overwhelmingly male, came from upper status positions and business (as opposed to labor) backgrounds, were less likely to be married, had preterrorist membership in a conventional political party, and were more likely to be in leadership than supportive positions inside terrorist organizations. An analysis of changes in the characteristics of right- and left-wing participants over time suggests that within both groups, terrorism spread through processes of both expansion (spread to progressively wider segments of the population) and diffusion (greater penetration within the same population segment). 6 tables and 8 notes.