This paper introduces the idea of the perceived curriculum from the student's viewpoint. Based on this, students' perception of opportunity to learn is measured and a process indicator developed for use in school effectiveness research and school evaluation. The Essential Skills of the New Zealand Curriculum are used as the foundation for the development of an instrument to measure opportunity to learn. The instrument measures students' perceptions of opportunity to learn from activities within the school environment. Some illustrative results from a baseline study of 1,300 pupils from 20 schools are presented. Potential uses for an instrument are proposed. The Essential Skills Access Test is appended. (Contains 3 figures and 2 tables.)