We present the precaution adoption process model--a stage theory consisting of seven distinct states between ignorance and completed preventive action. The stages are "unaware of the issue," "aware of the issue but not personally engaged," "engaged and deciding what to do," "planning to act but not yet having acted," "having decided not to act," "acting," and "maintenance." The theory asserts that these stages represent qualitatively different patterns of behavior, beliefs, and experience and that the factors that produce transitions between stages vary depending on the specific transition being considered. Data from seven studies of home radon testing are examined to test some of the claims made by this model. Stage theories of protective behavior are contrasted with theories that see precaution adoption in terms of movement along a single continuum of action likelihood.