This chapter discusses how systems can be described. As models are also systems, they can be described in the same way. We want to describe dynamic digital systems precisely and completely. Therefore, we discuss specifications and static and dynamic aspects of digital systems. First, we discuss languages and specifications in Sect. 4.1. We consider the semantics of specifications and the description of languages. Specification of static aspects is handled in Sect. 4.2 including perspective, system states, snapshots and system structure. Dynamic systems have more than one system state, and their state changes determine the system behaviour. Section 4.3 discusses the modelling of dynamics and behaviour including time, attributes, continuous and discrete behaviour as well as randomness.