This paper considers the standards-based assessment system used for high school science in New Zealand and explores the required assessment literacy of teachers working within this system, using a simple focus-area model. In New Zealand, school qualifications are achieved through the assessment of student work against a set of achievement standards, which are based on curriculum objectives. These standards define a broad range of science knowledge and skills expected of students across the three levels of New Zealand senior school qualifications. The socio-scientific focus of the science achievement standards means that students are required to investigate phenomena and demonstrate their ability to make meaning of data in a range of everyday contexts. Assessing student performance against these achievement standards requires considerable assessment literacy on the part of teachers, as the teachers are responsible for choosing or designing the assessment tasks as well as making the final summative judgements. Using the New Zealand qualification NCEA Level 1 Science as an example, this paper describes ways in which all teachers can approach the task of assessing students by focussing on five important areas concurrently. [A bibliography is included.]