Science assessment has been included with mathematics and language assessment on the international level since the 1970s. This paper discusses techniques of assessment that have been utilized to measure science process skills. The first section discusses the Assessment Performance Unit, a British project with the aim of developing innovative methods in assessing science achievement. Six categories of science activities for assessment purposes were identified by the project: (1) use of graphical and symbolic representation; (2) use of apparatus and measuring instruments; (3) observation; (4) interpretation and application; (5) planning of investigations; and (6) performance of investigations. The second section discusses the International Science Studies conducted by the International Association for Evaluation of Educational Achievement (IEA). Assessment items used for the first and second studies are described. The remainder of the paper presents the assessment techniques that were piloted for use in the third study. The following categories of questions were used in the Third International Math and Science Study: (1) multiple choice items; (2) open-ended written items; (3) performance tasks, which produce a physical product beyond writing; and (4) performance tasks where the process of actually doing the task is documented and examined. Sample items for each of these categories are presented and discussed. (Contains 16 references.) (MDH)