We survey the theory of equity in a variety of concretely specified resource allocation models: classical economies with private goods, economies with production, economies with indivisible goods, when monetary compensations are feasible and when they are not, economies with single-peaked preferences, and economies in which the dividend is a non-homogeneous continuum. We present the central fairness punctual notions, no-envy, egalitarian-equivalence, concepts of equal or equivalent opportunities and the relational principles of monotonicity and consistency.