Many children experience multiple family transitions as their parents move into and out of romantic relationships. The instability hypothesis is a stress mediation model that suggests that family transitions cause stress and that this stress leads to worse developmental outcomes. We conducted a systematic review to evaluate the evidence base for this hypothesis. Thirty-nine articles met the inclusion criteria. Most reports were secondary analyses of American longitudinal data sets. The support for the instability hypothesis was mixed, with many studies finding no evidence, or evidence only for certain groups, types of transitions, or outcomes. Protective factors and processes that prevent transitions from being stressful may explain some of the variability. Results suggest the need to empirically and theoretically differentiate relationship formation from dissolution, to examine effects of fathers' transitions, to include more and different types of outcomes, and to conduct this research within a broader variety of contexts. (Author abstract)