  Binary neutron star mergers observations are a unique way to constrain
fundamental physics and astrophysics at the extreme. The interpretation of
gravitational-wave events and their electromagnetic counterparts crucially
relies on general-relativistic models of the merger remnants. Quantitative
models can be obtained only by means of numerical relativity simulations in 3+1
dimensions including detailed input physics for the nuclear matter,
electromagnetic and weak interactions. This review summarizes the current
understanding of merger remnants focusing on some of the aspects that are
relevant for multimessenger observations.
