For the past 30 years, school-based assessment (SBA) has been a major feature of GCSEs and A levels, the main school examinations in England. SBA has allowed teachers to allocate marks to their students for the level of skills that they show in their work. Such skills include for example, experimental techniques in science, performance in drama and enquiry skills in history. These skills can be difficult to assess validly in written examinations. Of course, SBA can also provide an alternative form of assessment for the same knowledge and skills from timed, written examinations taken at the end of the course of study. At the start of the millennium, concerns grew that plagiarism, excessive input from teachers and parental support were distorting SBA marks and adversely affecting learning. Attempts were made to tighten the arrangements around SBA but in the context of England’s school accountability arrangements; these did not prove wholly successful. The current, substantial reforms have considerably reduced the use of SBA, entirely removing it in some subjects, relying much more on exams. This paper describes the influence of school accountability arrangements on the design of new GCSEs and A levels and includes evidence from a teacher survey of assessment practices in schools. It explores the principles by which decisions have been made regarding the assessment arrangements determined for different subjects. It considers how the reduction in SBA might have a positive influence on the taught curriculum.