Shulman’s Pedagogical Content Knowledge (PCK), developed in the mid-1980s has undergone a number of revisions, with the aim of building upon the concept. The current paper introduces a new theoretical concept named Assessment Pedagogical Content Knowledge (APCK), for implementing formative assessment during science teaching in order to aid students’ learning. The new concept, APCK, is an amalgam of knowledge that draws on how to combine assessment with pedagogy. It involves a combination of knowledge of assessment, the skills of assessment and the contents that are taught. The appropriate use of APCK inures to the benefit of students’ science learning. The study uses both qualitative and quantitative approaches to show that integration of assessment with instruction has proved valuable for science learning. The feedback, which teachers give to their students during teaching are used by students to improve their learning of science. This shows that a good grasp of the concept will enable teachers to use APCK formatively when teaching science and their students will improve their science learning. Hence, APCK holds a great potential, especially for science teachers, to develop their skills of interweaving instruction with assessments to promote learning. This study then proposes that APCK of science teachers can be identified through their knowledge and practice of the inter-relationships between pedagogy and assessments.