The notion of natural design is supposed to do some important work in philosophy of biology: to explain why functional language is adequate in biology, though superfluous in physics. Most attempts to explicate the notion of design as a basis for a definition of biological function result in an explanatory circle. In contrast, my own explication allows, for the first time, for a non-circular definition of function by reference to design. Like with earlier attempts, the straightforward application of these concepts is the reconstruction of biological theories that are based on genetic determinism. However, the application of the concepts is not restricted to gene-centered theories. They can be used to investigate the various roles that function ascriptions play in different biological theories. In this project, I will apply the explication of biological design to developmental aspects of biological function, as put forward in the approaches of developmental systems theory (DST) and evolutionary developmental biology (EvoDevo). My aim is a comparison of the role that function ascriptions play within the different theoretical frameworks, as well as a survey of possible justifications for the use of specialized concepts of function in these approaches. To follow this goal, I will first reconstruct biological theory elements (models) that belong to the approaches mentioned. Reconstructions will be based on Lakatos’s methodology of scientific research programs and on an informal variation of the structuralist approach of model theory. While the first method will show the status of function ascriptions within the research programs, the second will be used to investigate structural differences between the biological theories. In a next step, not only isolated models but the linkeage of models belonging to the same research program will be reconstructed. The reconstruction of these so-called theory nets shall yield insights into the possibilities and constraints of the explanatory powers of the different research programs with respect to function ascriptions. Within my reconstructive and descriptive approach, this will not result in an assessment of the research programs. However, the results may be used within the biological approaches to formulate criteria for such an assessment.