  In this talk I discuss three main topics concerning the theoretical
description and observable signatures of possible phase transitions in nuclear
collisions. The first one is related to the multifragmentation of thermalized
sources and its connection to a liquid-gas phase transition in finite systems.
The second one is dealing with the Coulomb excitation of ultrarelativistic
heavy ions resulting in their deep disintegration. The third topic is devoted
to the description of a first order phase transition in rapidly expanding
matter. The resulting picture is that a strong collective flow of matter will
lead to the fragmentation of a metastable phase into droplets. If the
transition from quark-gluon plasma to hadron gas is of the first order it will
manifest itself by strong nonstatistical fluctuations in observable hadron
distributions.
