For some people involved in certain relationships, the transition to parenthood is associated with precipitous declines in marital satisfaction and increases in depressive symptoms. After briefly reviewing the transition to parenthood literature, we explain why attachment theory is particularly well suited to account for how and why certain people involved in certain relationships are so vulnerable to these negative outcomes. Testing diathesis-stress models, we present evidence from our own research indicating that more anxiously attached women who enter parenthood perceiving lower levels of spousal support or greater spousal anger are especially susceptible to experiencing significant pre-to-postpartum decreases in marital satisfaction and increases in depressive symptoms across the first 6 months of the transition. We also present evidence isolating some of the psychological processes that may be partially responsible for generating and sustaining these negative outcomes. We conclude by discussing how specific prevention programs, experimental interventions, or individually based therapies could be developed to buffer vulnerable people from negative postpartum outcomes.