This paper, published in one of the top journals in Computer Science, describes the UPML framework for modelling knowledge-based applications, which can be seen as the culmination of over twenty years of research in this area. UPML has shaped the field, providing the intellectual and formal basis for the WSMO framework, which is the cornerstone of current standardization efforts in semantic web services and has attracted over E100M in funding from EU. Since 2001, the Open University itself has received over £3.5M in funding from EU/EPSRC to carry out further research in models of problem solving and semantic web services.