Educators in many countries around the world have a strong interest in improving early childhood mathematics education, one component of which is formative assessment. Unlike summative assessment, this approach can provide teachers with information useful for understanding and teaching individual children. This paper describes the rationale for and the development of the birthday party (BP), a mathematics assessment system appropriate for investigating the mathematical thinking and learning of young children, 3-, 4-, and 5-years of age. The BP uses computer technology to help teachers administer the assessment. The system also provides professional development to help teachers understand the results of the assessment and determine how to promote individual children's learning. The ultimate goal of the system is to nurture thoughtful teachers who can conduct inventive formative assessments on a daily basis without the guidance of the BP software.