A recent UK House of Commons report on Science 14-19 identified problems with coursework and argued for a greater emphasis on teaching and assessment of scientific literacy. This paper describes a written test for procedural understanding, given to 15 year olds, that addresses both of these issues. Comparisons are made between the scores on a written test of procedural understanding with both assessments made of subject knowledge and pupil accounts of investigations. The potential advantages of assessing procedural understanding by written tests are discussed.