Summary: In about 380 words the entry introduces the principle of proportionality as a decision- making and as a representational principle, and traces in detail the development of the term from the 18th century. Proportionality as a decision-making principle refers to the distribution of positions according to party or confessional membership which involves bargaining and compromise. In political systems with proportional representations, mandates are distributed according to the relative electoral success of the parties which each vote nearly equal weight. Proportionality as representational principle refers to the goal that the parliament should resemble as closely as possible the socio-cultural make-up of the electorate. The entry proceeds by reviewing the historical development of mathematical methods to translates votes into mandates, and by reviewing the origins of the representational principle of proportionality.