Classroom assessment is a critical aspect of teaching and learning. In this paper, Taiwanese high school students' conceptions of science assessment and the relationship between their conceptions of science assessment and of science learning were investigated. The study used both qualitative and quantitative methods. First, 60 students were interviewed regarding their conceptions of science assessment. From these interviews, a survey was developed, validated, and used with 914 students to quantitatively examine the same question. Third, 224 students were surveyed to determine the relationship between their conceptions of both science assessment and science learning. Using the phenomenographic method, the first study revealed six categories of conceptions of science assessment: reproducing knowledge, rehearsing, accountability to learning, improving learning, problem solving, and critical judgment, which seem to reveal the surface, summative, and formative purposes of assessment, respectively. The second study, based on what the high school students reported about science assessment, resulted in the use of the Conceptions of Science Assessment questionnaire, which confirmed the second-order analysis (surface, summative, and formative) of the purposes of science assessment. In the third study, the relationships between high school students' conceptions of science assessment and of learning science were revealed. (Contains 7 tables and 1 figure.)