Authentic observation is a type of assessment that is well aligned with learning through play and everyday situations. Studies in developmental psychology based on clinical psychometric testing indicate that young children’s understanding of counting and cardinality is crucial for their further learning of mathematics in school; however, psychometric testing is incompatible with play-based learning. This study explores whether and how assessment of cardinality competence of children from 3–5 years of age can be satisfactorily achieved via authentic observation and is based on analysis of interview data from a project where kindergarten staff used observation materials with the intention of authentic observation and on document analysis of tasks and materials designed for the assessment of children’s cardinality competence. It is discussed how appropriate tasks are for authentic observation and whether the proposed authentic observation materials sufficiently assess the same cardinality competence as corresponding developmental psychology tests.