Given the centrality of sample selection bias treatments in mergers and acquisitions (M&A) analyses, a periodical stocktaking of our theoretical and empirical knowledge is necessary. This paper describes the sources of selection bias and its implications for inference in M&A analyses. Selection bias arises not only exogenously, but also endogenously due to the complexity of the underlying economic phenomenon. The Heckman model is reintroduced with M&A-specific intuitions, and several empirical considerations are explained. A few powerful variations of the baseline model are discussed. The goal is to help M&A researchers to apply the Heckman procedure, assess its empirical validity, and interpret it correctly.