Unexpected, non-normative events are key influences on human development across the life span. Despite this importance, little is known about how an individual may capitalize on unexpected events and transform them into opportunities for sustained positive development. In this article, to address this theoretical lacuna, I introduce the concept of serendipitous relations - mutually beneficial, adaptive developmental regulations brought about by the time-extended coaction of intentional self-regulatory actions and unexpected non-normative life events. I enumerate five specific intentional self-regulatory serendipitous actions hypothesized to lead to serendipitous relations. Using developmental theory, life examples, and examples from the literature, I hypothesize that serendipitous relations brought about by effective use of serendipitous actions may be important sources of adaptive development. I conclude by offering specific suggestions for future research on serendipitous relations across the life span.