By the term ?professional end user developers? we mean people such as research scientists who work in highly technical, knowledge-rich domains and who develop software in order to further their professional goals. In common with other end user developers, professional end user developers do not describe themselves as software engineers and have no formal training in software engineering. They differ from most other end user developers, however, in that learning programming languages rarely presents them with any problem. In this paper, drawing on data from field studies of different groups of professional end users, we examine the problems that such people face in meeting the demands of software development given the culture in which they work and their normal development practice. Understanding these problems is an essential prerequisite to developing tools, techniques etcetera to support professional end user development.