The Unified Modeling Language (UML), proposed by the Object Management Group (OMG), is a general purpose modeling language which became the standard for modeling system’ structure and behaviour. A UML model offers different views of the system in the form of various diagrams. The talk’s focus are UML State Machine Diagrams, widely used to specify dynamic systems behaviours. The official semantics of UML State Machine Diagrams (and UML in general) is described in a semi-formal manner where some bits are deliberately left undefined allowing different semantic interpretations. In this talk, an overview of UML State Machine Diagrams is given and some of the semantics ambiguities are pointed out, describing how they have been faced and implemented in the literature.