In this thesis we introduce definitive (definition-based) programming, and examine its applicablity to both parallel synchronous programming and modelling and synchronisation of concurrent systems. We show that definitions can be used to program in a highly parallel but interference-free manner, by taking advantage of the synchronous nature of operation of our model of computation, the abstract definitive machine (adm). We introduce a cognitively-based definitive specification notation called LSD, which allows modelling of concurrent systems in a natural manner. An LSD specification can be transformed into a family of adm programs, to allow simulation of the behaviour described by the specification. This strategy combines the cognitive expressiveness of LSD and the computational power of the adm.