There is perhaps no subject more contentious in schools than assessment and yet, often, at classroom, school and national level, inferences and decisions are made without much reference to research in this area. In fact, teachers often accept or interpret assessment requirements without question, feeling that assessment has to be approached in a specific way rather than considering how assessment might be best used to promote learning. This is not surprising, as development in the area of assessment often dwells on comparisons of standards with previous regimes and by its very nature becomes conservative. Also, the introduction of national assessments in 1989, accompanied by published league tables of school examination performance, has focused many schools to consider assessment from a performance orientation rather than a learning orientation, so that checking on performance prior to the actual examinations has been concentrated upon, rather than how assessment affects learning in the classroom. This article attempts to provide a rough guide across part of the field of educational assessment research to help teachers think about how and why they use particular assessments, in the hope that it will allow science teachers to strengthen their assessment practices.