The aim of the research was to investigate the influences on construction professionals' working practices. The existing literature in the field is very dispersed dealing with aspects of professionalism (sociology), the acquisition and use of knowledge (cognitive psychology) and contractual obligations and responsibilities (law). From the literature review the concept of Professional Finns in Construction (PFCs) is developed. Four PFCs were used in the research; architects, environmental engineers, quantity surveyors and structural engineers. Within each PFC four hierarchy levels were identified; strategic (highest), general, operation and direct work (lowest). The developed methodology was to carry out case studies of four building projects which provided data on the influences of each of the four hierarchy levels within one of the four PFCs. This approach allowed standardised data to be collected. The influences were identified by means of content analysis of interviews carried out with the construction professionals engaged on the case studies. Altogether fifty-eight construction professionals were interviewed. These interviews were tape recorded, transcribed and analysed utilising a computer software program called HyperResearchTm. From this analysis twelve influences were identified, each influence having three subcategories of positive, neutral and negative. The twelve influences are grouped into three categories of client-based, project-based and non-specific and are represented on charts relating them directly to the interviewee who exhibited them. From over ninety-five hours of transcribed interviews analysed a total of one thousand five hundred and thirty-eight instances of influences were exhibited by the fifty-eight construction professionals. Supporting data for the influences exhibited during the interviews were collected and analysed for each of the fifty-eight construction professionals concerning their time involvement in a project, previous interactions with other members of the project team and the contractual obligations and relationships of their employing PFC. The findings of the research show different influences according to the type of PFC and the hierarchy level within the PFCs. Architect, environmental engineer and structural engineer PFCs are more likely to be influenced by the particular circumstances of a project than a quantity surveyor PFC. All PFCs are equally influenced by the client. Construction professionals at the strategic and general hierarchy levels are more influenced by the client than those at the operation and direct work hierarchy levels. Construction professionals at the operation and direct work hierarchy levels are more likely to be influenced by the project than those at the strategic and general hierarchy levels.