Intermetallic γ-TiAl based alloys are innovative structural materials dedicated to applications in the automotive and aeronautic industries. Especially their low density, high specific yield strength, and excellent resistance against creep and oxidation make them a suitable choice for structural high-temperature components in combustion engines. However, further improvement of their properties and processability is required to conquer new application areas and facilitate cost-effective production. While the incorporation of additional alloying elements is a promising possibility, their effects on the phase transformations and phase equilibria need to be considered rigorously. In this context, this review provides a detailed survey of the research work on the influence of technically important alloying elements on the Ti–Al phase diagram. First, an introduction to the fundamentals of the phase transformations in γ-TiAl based alloys and the consequences of changed cooling conditions, relevant for example to the latest developments within the field of processing, is given. Afterward, the alloying elements, categorized with respect to their stabilization effect, are discussed and their particularities are highlighted. Topics covered include established ternary phase diagrams and isoplethal sections, the stabilization of additional phases as well as the influence of alloying elements on the microstructure of modern engineering intermetallic γ-TiAl based alloys.