The traditional software development paradigm, the waterfall life cycle model, is defective when used for developing real-time systems. This thesis puts forward an executable prototyping approach for the development of real-time systems. A prototyping system is proposed which uses ESML (Extended Systems Modelling Language) as a prototype specification language. The prototyping system advocates the translation of non-executable ESML specifications into executable LOOPN (Language of Object Oriented Petn Net) specifications so that ESML can be used as a graphical executable specification language for the prototyping of real-time systems. If the translation process is automatic then the user need not be aware of LOOPN. The ESML/LOOPN prototyping system defines an execution semantics for the ESML language in terms of LOOPN nets, a set of translation templates are supplied for the translation of ESML language specifications into LOOPN language specifications. The execution semantics are based on a set of execution rules (guidelines) which have been defined for ESML to allow prediction of the behaviour of ESML specifications over time. A C language program which can be run by the user as a prototype of the modelled system is generated automatically from the LOOPN specification. The ESML/LOOPN prototyping system has been applied to build an exploratory prototype of a typical real-time system, 1e the Fuel Subsystem of the Auxiliary Power Unit (APU), an avionic system used on the Boeing-737 airplane series.