Contemporary curricula are increasingly focused on broader framings of student outcomes such as 'competencies', 'capabilities' and '21st century skills', as we realise that declarative disciplinary knowledge no longer, on its own, confers the capacity to deal with the complex and fast-changing world that students are now entering. In mathematics, the Australian curriculum specifies a set of 'proficiencies' -- understanding, fluency, problem-solving and reasoning. There is no parallel set of proficiencies for science, although there have been attempts to specify these in international assessments. In this paper, we argue the case for a set of science proficiencies and describe such a set that was developed to underpin and enrich a major professional learning initiative of the Victorian Department of Education and Training (DET). We unpack the nature of these proficiencies and their relations -- "understanding," "investigating," "interpreting evidence," "linking science" and "society," and "productive disposition" -- and describe their capacity to enrich curriculum planning. Finally, we discuss the assessment of these proficiencies and explore the possibility of establishing their developmental progression.