The importance of assessing the sustainability of companies and their activities is increasing. Especially since the implementation of the new Corporate Social Reporting Directive, even more companies are committed to reporting on the impacts of their businesses on sustainability. This is a challenge especially concerning the social dimension of sustainability. Some frameworks present various relevant social criteria that can be used as a basis for assessment. However, these do not provide detailed suggestions for handling the numerous methodological challenges of such a multi-criteria assessment: identification and selection of the relevant stakeholders, categories and indicators to measure the impacts on social sustainability, weighting and aggregation of these criteria, etc. Therefore, this paper contributes to the methodology of social sustainability assessment by presenting a procedure model for this specific assessment task. The novelty of the model results from its foundation by a review of methods for selection, normalisation, weighting and aggregation of social criteria, existing decision theory-based procedure models, as well as a stakeholder-oriented catalogue of criteria. The procedure model is structured hierarchically by subdividing the overall social sustainability-assessment task into different levels: stakeholders, categories, and indicators. Furthermore, appropriate methods are suggested for the single steps of the procedure model.