Serendipity, the notion of researchers making unexpected and beneficial discoveries, has played an important role in debates about the feasibility and desirability of targeting public R&D investments. The purpose of this paper is to show that serendipity can come in different forms and come about in a variety of ways. The archives of Robert K Merton, who introduced the term to the social sciences, were used as a starting point for gathering literature and examples. I identify four types of serendipity (Walpolian, Mertonian, Bushian, Stephanian) together with four mechanisms of serendipity (Theory-led, Observer-led, Error-borne, Network-emergent). I also discuss implications of the different types and mechanisms for theory and policy.