This collection of reprints describes a unified treatment of semantics, covering a wide range of notions in parallel languages. Included are several foundational and introductory papers developing the methodology of metric semantics, studies on the comparative semantics of parallel object-oriented and logic programming, and papers on full abstraction and transition system specifications. In addition, links with process algebra and the theory of domain equations are established. Throughout, a uniform proof technique is used to relate operational and denotational models. The approach is flexible in that both linear time, branching time (or bisimulation) and intermediate models can be handled, as well as schematic and interpreted elementary actions. The reprints are preceded by an extensive introduction surveying related work on metric semantics.