In this paper, we propose a knowledge-based approach to design lower limb prostheses; in particular, we focus on the 3D modelling of the socket, the most critical component. First, the architecture of a dedicated design framework is described, detailing features of the main design steps. Then, the paper discusses the acquisition and formalisation of the knowledge related both to the prosthesis manufacturing process and to the considered component. Finally, we present the computer-aided module, named socket modelling assistant-SMA, we specifically developed to design the socket. It is a virtual laboratory where the socket virtual prototype is generated directly on the digital model of patient’s residual limb. It guides and supports the designer during each step in an automatic and/or semi-automatic way applying design rules and procedures. The modelling steps and available interactive tools that emulate orthopaedic technician’s operations are described. Results of the experimentation phase are described. At current state of the prototype development, they are encouraging and have permitted to preliminarily validate the proposed approach and envisage future improvements.