Neutron star mergers and core-collapse supernovae are connected to some of the most pressing open questions in physics and astrophysics, ranging from the nature of strong gravity, to the physics of dense matter in neutron stars, to the origin of the heavy elements, to the mechanism powering gamma-ray bursts. Multimessenger observations of these events hold the key to unlock these mysteries. However, theory is needed to turn data into answers. In this talk, I will discuss our efforts to model neutron star mergers and stellar explosions in numerical relativity. I will talk about new developments in the simulation technology and recent results. Finally, I will talk about challenges and prospectives in this field.