As part of a major review of teachers' professional conditions of service in Scotland the "Chartered Teacher Programme" has been introduced offering experienced teachers the option of a substantial salary enhancement, not by promotion to a school management role, but by staying in the classroom and engaging in a programme of professional development, pursuing an accredited professional qualification (Chartered Teacher status) and a post-graduate masters degree. The level of accomplishment required is assessed against the "Standard for Chartered Teacher" (SCT). The present article examines the steps taken to ensure that this standard was valid, grounded and empirically well justified. The SCT was derived by means of indepth interviews with accomplished teachers, focus groups involving teachers, teacher educators, parents, children, education managers and school inspectors, as well as two national consultation exercises. Analysis of the accumulated data yielded a model of accomplished teaching, comprising four principal elements: "Professional Values and Commitments, Professional Knowledge and Understanding, Professional and Personal Attributes and Professional Action". The implications of this model for teacher assessment in the context of Continuing Professional Development (CPD) are discussed with reference to a sample of the assessment tools being used within the new modular Chartered Teacher Programme in Scotland. The assessment tool of preference is the portfolio incorporating an element of action research assessed against criteria directly derived from SCT. What emerges is the need for ownership of the assessment process to be genuinely shared among stakeholders, with the emphasis on professional self-evaluation, but also with the added rigour of academic validation and external professional accreditation.