We present a formalism for modeling replication in a distributed system with concurrent users sharing information. It is based on actions, which represent operations requested by independent users, and constraints, representing scheduling relations between actions. The formalism encompasses semantics of shared data, such as commutativity or conflict between actions, and user intents such as causal dependence or atomicity. It enables us to reason about the consistency properties of a replication protocol or of classes of protocols. It supports weak consistency (optimistic protocols) as well as the stronger pessimistic protocols. Our approach clarifies the requirements and assumptions common to all replication systems. We are able to prove a number of common properties. For instance consistency properties that appear different operationally are proved equivalent under suitable liveness assumptions. The formalism enables us to design a new, generalised peer-to-peer consistency protocol.