The advent of coupled earth system models has raised an important question in parallel computing: What is the most effective method for coupling many parallel models to form one high-performance coupled modeling system? We present our solution to this problem--The Model Coupling Toolkit (MCT). We describe how our effort to construct the Next-Generation Coupler for NCAR Community Climate System Model motivated us to create the Toolkit. We describe in detail the conceptual design of the MCT, and explain its usage in constructing parallel coupled models. We present some preliminary performance results for the Toolkit's parallel data transfer facilities. Finally, we outline an agenda for future development of the MCT.