The effectiveness of the prevention through design (PtD) approach to the management of health and safety (H&S) risks on construction sites is widely acknowledged. This approach underlies the construction, design, and management (CDM) Regulations in the United Kingdom that provide for a Principal Designer (PD) role with a statutory duty to plan, manage, and monitor the preconstruction phase of projects and to coordinate matters of H&S during that phase. Although there is a growing body of research literature on PtD practice, there is a gap in the general issue of the practical implementation of the CDM Regulations in general and the performance of the PD in particular. The purpose of this paper is to report research undertaken with the aim of plugging this gap. The part examined concerns the challenges that beset the performance of the PD role and the drivers behind such barriers. A qualitative research design was adopted using, for data collection, 14 focus group discussion sessions involving 89 participants with direct experience of practical implementation of the regulations. The research uncovered three broad categories of barriers to the performance of the PD role: inadequacies in the client’s general approach to its duties; supply chain fragmentation and insurance challenges; and performance-related challenges stemming from limitations in PD technical competence and interpersonal skills. The research is the first study focused on the H&S risk management processes and the organizational and operational barriers to effective management and coordination of H&S matters by PDs. The research outcomes are of obvious relevance to H&S management practice in not only the United Kingdom but also European Union countries and other countries with similar regulations. As management of design H&S risks at the preconstruction stage is an inherent feature of the PtD concept, they could also inform PtD practice with respect to coordination of the work of the different design specialisms involved.