A welfare assessment system should reflect basic considerations concerning the definition of the term 'welfare' and the goal of the welfare assessment. The aggregation of welfare indicators into an operational welfare system is described as a process in which indicators are evaluated and re-evaluated step by step for their independent welfare relevance, marginal information value and suitability for use in on-farm studies. Relevant indicators are aggregated into a protocol, and finally this protocol is evaluated in terms of relevance as a welfare assessment system. The theory is illustrated by an example: 'Human-animal relationship among loose housed pregnant sows'.