Thermalization is the process by which a typical closed quantum system in a generic initial state approaches local thermal equilibrium. Its occurrence precludes some fascinating and potentially useful equilibrium states of matter. However, under certain conditions, thermalization only occurs after an exponentially long time. During the intervening time interval, which is called the prethermal regime, most of the beneficial properties associated with thermal equilibrium continue to hold, but some of the limitations are removed. As a result, states of matter such as time crystals and self-correcting quantum memories can occur.