The benefits of close relationships for mental and physical health are well documented. One of the mechanisms presumed to underlie these effects is social support, whereby close others provide practical and emotional assistance in times of need. Although there is no doubt that generalized perceptions of support availability are beneficial, research examining actual instances of support receipt has found unexpectedly mixed results. Receiving support sometimes has positive effects, but null or even negative effects are common. In this article, we review our multimethod program of research that seeks to understand and explain the costs of receiving social support. We focus on reductions in the recipient's sense of relationship equity and self-efficacy as mechanisms of this effect and examine a number of other moderating factors. Although we have found that receiving support incurs costs on average, there is considerable variability yet to be explained. Using diary data from 312 persons preparing to take a challenging exam, we examined the potential of individual differences in neuroticism, agreeableness, and attachment insecurity to explain variability in experienced support costs. We close with new questions about why received support may be beneficial or benign in some situations while being especially toxic in others.