This article describes 2 studies that examined changes in psychosocial work characteristics (job strain model) and health-related behaviors as potential explanations of the job insecurity-health relationship in a longitudinal cohort of white-collar British civil servants. Job insecurity arising from anticipation of change was associated with a modest increase in self-reported morbidity, whereas chronic job insecurity was associated with some adverse physiological changes. Anticipation of change and chronic job insecurity were associated with adverse changes in other psychosocial work characteristics, but few changes were significant and consistent across both exposure groups. Changes in health-related behaviors associated with either exposure were slight. Apart from a minor role for social support at work in both sexes and a modest role for job demands in women, adverse changes in these factors explain little of the job insecurity-health relationship.