The task of a high-level behavioural synthesis system is to create a structure to implement a given abstract specification of behaviour. The behaviour is specified at the algorithmic level, typically in the form of a high-level programming language. The synthesized structure is described at the register-transfer level. In such systems the synthesis task is guided only by the behaviour and some physical design constraints such as speed and area. This approach frequently leads to difficulties in synthesizing a suitable architecture. The synthesis system reported in this thesis tackles this problem by enabling the designer to specify structural input alongside the behaviour. The structural input is described at the register-transfer level and need not define a complete structure. The synthesis tool makes use of this input structure by incorporating it into the design where appropriate or as instructed. This structurally directed approach is shown to give the designer greater control over structural aspects of the design in addition to enabling greater exploration of possible structural solutions.