The present research focuses on the psychometric properties of the Birthday Party measure for ages 3-5. The Birthday Party was developed to provide a reliable, valid, and engaging measure of early mathematical content--Number and Operation, Shape, Space, and Pattern--that can be given in either a short or a long form to English and Spanish speakers. 522 preschoolers (147 for age three, 226 for age four, and 149 for age five) were tested with both the Birthday Party and a criterion measure: the mathematics portion of the Young Children's Achievement Test to examine reliability and validity. Overall, the technical adequacy of the Birthday Party was established as follows: (1) the results indicated that the Birthday Party was reliable and showed evidence to support both criterion and construct validity, (2) the Spanish version of the Birthday Party displayed adequate reliability and validity results, as well as equivalence with the English version of the Birthday Party, and (3) a short version of the Birthday Party is available as a screening measure to help aid in the early identification of children who are weak in foundational mathematical skills and add to the Birthday Party's practicality and usability. (author abstract)