This book presents theoretical methods and experimental results on the study of multipartite quantum correlations in spin-squeezed Bose-Einstein condensates. Nonclassical correlations in many-body system​s are particularly interesting for both fundamental research and practical applications. For their investigation, ultracold atomic ensembles offer an ideal platform, due to their high controllability and long coherence times. In particular, we introduce criteria for detecting and characterizing multipartite entanglement, Einstein-Podolsky-Rosen steering, and Bell correlations. Moreover, we present the experimental observation of such correlations in systems of about 600 atoms.