Modal synthesis is an important area of physical modeling whose exploration in the past has been held back by a large number of control parameters, the scarcity of general-purpose design tools and the difficulty of obtaining the computational power required for real-time synthesis. This paper presents an overview of a flexible software framework facilitating the design and control of instruments based on modal synthesis. The framework is designed as a hierarchy of polymorphic synthesis objects, representing modal structures of various complexity. As a method of generalizing all interactions among the elements of a modal system, an abstract notion of {\it energy} is introduced, and a set of energy transfer functions is provided. Such abstraction leads to a design where the dynamics of interactions can be largely separated from the specifics of particular modal structures, yielding an easily configurable and expandable system. A real-time version of the framework has been implemented as a set of C++ classes along with an integrating shell and a GUI, and is currently being used to design and play modal instruments, as well as to survey fundamental properties of various modal algorithms.