There are only a few instruments to assess mathematics knowledge and skills in children as young as three to four years of age, and these instruments are limited in scope of content. We describe the development of a theoretically based, empirically tested instrument designed to measure the mathematical knowledge and skills of children from three to seven years of age, emphasising its submission to the Rasch model. After using the data to refine the instrument, they fit the model well, with high reliability. These data also provided empirical support for the developmental progressions for most topics. We conclude with a description of the research's contribution to theory and empirical research regarding young children's development of specific mathematical competencies. (Contains 2 figures and 3 tables.)