The formation of infectious virus particles is a highly complex process involving a series of sophisticated molecular events. In most cases, the assembly of virus structural elements results in the formation of immature virus particles unable to initiate a productive infection. Accordingly, for most viruses the final stage of the assembly pathway entails a set of structural transitions and/or biochemical modifications that transform inert precursor particles into fully infectious agents. In this chapter, we review the most relevant maturation mechanisms involved in the generation of infectious virions for a wide variety of viruses.