The concept of activity is discussed briefly. Emphasis is placed on the principles of activity measurement and recent experimental developments as well as on temperature and concentration dependences, thermodynamic modeling, and modern thermodynamic evaluation. The activities as well as mixing properties of all types of alloys, even including metallic melts showing short-range ordering, can be represented algebraically by means of a simple concept. Using just one common set of parameters allows a straightforward description of combined temperature and concentration dependences. An extended temperature dependence of the excess Gibbs energy, as necessary for this purpose, has been derived. The presented concept makes possible putting together all activity measurements of a multicomponent alloy system for performing one algebraic overall best-fit. In connection with a personal computer this forms a very convenient, interactive evaluation tool yielding a considerable reduction of both, the experimental work and the time spent on evaluation.