Most engineering problems today are multi-disciplinary. Security issues, environmental concerns, rising consumptions, depleting resources and globalization are issues that impose the multi-disciplinary task. The mathematical formulation of the communication among participating disciplines should deserve special attention. In this paper, a framework for handling the communication in multi-disciplinary problems in general is established in terms of a purpose of analysis and a protocol for the flow of information. The formulation that evolved from the optimal design of an energy-conversion system given a cost objective function is reviewed. The focus is then set on the communication between the disciplines of thermodynamics and design and on the mathematical formulation of the communication.