Realizability -checking whether a specification can be implemented by an open system -is a fundamental step in the design flow. However, if the specification turns out not to be realizable, there is no method to pinpoint the causes for unrealizability. In this paper, we address the open problem of providing diagnostic information for realizability: we formally define the notion of (minimal) explanation of (un)realizability, we propose algorithms to compute such explanations, and provide a preliminary experimental evaluation.