Prokaryotic Class 2 CRISPR-Cas systems mediate adaptive immunity against invasive genetic elements by means of standalone effector proteins that function as RNA-guided nucleases. The effectors Cas9 and Cas12 generate double-strand breaks in DNA substrates, which has been exploited for genome editing applications. In turn, Cas13 enzymes function as RNA-guided ribonucleases whose non-specific activity is triggered by target RNA binding. In this review, we highlight recent structural investigations of Cas9, Cas12 and Cas13 nucleases that have illuminated many aspects of their molecular mechanisms. In particular, these studies have highlighted the role of guide RNA seed sequences in facilitating target recognition and the importance of conformational transitions in controlling target binding and cleavage.