Research aimed at developing and validating an instrument to assess secondary school students' perceptions of assessment tasks was conducted. Following a review of literature, a five-scale instrument of 40 items was trialled with a sample of 658 science students in 11 English secondary schools. Based on internal consistency reliability data and exploratory factor analysis, refinement decisions resulted in a five-scale instrument called the "Perceptions of assessment tasks inventory" (PATI). The scales of the PATI are Congruence with planned learning, Authenticity, Student consultation, Transparency and Diversity.