Distribution systems are naturally unbalanced as a consequence of unbalanced loads and the electrical network itself, where the use of single-phase or two-phase sections is not uncommon. In addition, a detailed representation of a typical distribution system can have tens of thousands of nodes, depending on the area selected for study and whether a three-phase representation is employed. This contrasts with Transmission systems, where typical sizes in terms of number of nodes are much smaller and where a balanced (single-phase) representation is sufficient in most situations. Thus, Transmission and Distribution systems have traditionally been studied separately, but this approach is becoming increasingly inadequate for various reasons. This paper then proposes a methodology that integrates Transmission and Distribution systems into a common electrical model. At the core of the methodology there are 1) single-and three-phase models for each component (branches, transformers, loads, etc.) and 2) interchangeable balanced and unbalanced network equivalents that allow for analyzing specific areas of a given complete network. Potential gains brought about by the proposed methodology are illustrated through its application in a real-world electric system.