This article argues that whilst there have been substantial advancements in the ways in which learning is conceptualised, theoretical understandings of assessment, and practices that contribute to meaningful statements of learners' achievements, have not mirrored these changes in certain learning contexts. The authors challenge contemporary methods of educational assessment, particularly for marginalized groups of learners, specifically young people and adults in informal, vocational and professional learning. They critique the assessment and formal accreditation opportunities available to these groups of learners, and emphasise a need for more authentic, learner-friendly methods to encourage their engagement and retention. Drawing on empirical work with three contrasting groups of learners, they outline a theoretical framework for assessing the authentic performances of learners in a range of settings. The assessment of authentic performance is an important but under-reported area of assessment activity, which has the potential to address a number of contemporary criticisms of assessment systems in national and international contexts. The emphasis on the assessment of performance in this article recognises that informal and vocational learning is inherently social, is mediated by more knowledgeable others and relies on professional judgements as to how good practice is defined, shared and understood within the wider community. The first section sets out this theoretical framework alongside the "assessment problem", and defines the authors' understanding of "authentic assessment". The second section describes the outcomes of an empirical study that aimed to develop processes and rubrics of alternative forms of assessment based on performance. The conclusion proposes some key components of a model for the authentic assessment of performance. (Contains 3 notes.)