The paper studies the problems regarding design of moulds for injection moulding of thermoplastics. A brief overview is given of the new trends in the field of mould design and manufacturing. It also deals with the problems of mould development strategy, focusing on defining of moulding and mould families according to the principles of similarity, thus providing possibilities of mould design variations. The mould functions and subsystems are described in detail, with special consideration of the mould design process analysis including conceptual mould design, mould elements dimensioning, and mould design documentation development. Each of these mould development phases is presented as a system by applying basic concepts of systemic technology theory, thus defining the systemic models of individual mould development phases. Using the systemic models of mould development offers better insight in the mould design process itself, and the exact sequence of activities is defined. Within the conceptual mould design, the use of decision-making diagrams is suggested which provides design solutions of certain partial mould functions. The existing decisionmaking diagrams have been checked on concrete examples, and their analysis has led to corrections. During the phase of mould components dimensioning, an overview of analytical and numerical rheological, thermal and mechanical mould calculations is given. The possibility of using computers in mould development has been analysed with special consideration. The paper provides an example of designing a mould according to pre-defined guidelines, comparing the procedure and the results of analytical and numerical mould calculation.