Authentic assessment "refers to the systematic collection of information about the naturally occurring behaviors of young children and families in their daily routines" (Neisworth & Bagnato, 2004, p. 204). In formative assessments, the assessment information informs instruction. Formative assessments are given periodically and should be used to make ongoing curriculum and program changes for a child, or groups of children (Cook, Klein, & Tessier, 2008). Developmentally appropriate practices incorporate systematic planning for curriculum and assessment reflecting a child's progress (Copple & Bredekamp, 2009). Curriculum-based assessments are informal assessment procedures that monitor students' progress (Deno, 2003a). The mathematics assessments discussed in this article are considered informal for the following reasons: (1) the preschool teachers created the assessments in collaboration with the university professors; (2) the assessments are tied to the classroom curriculum and mathematics; and (3) the assessments were used for progress monitoring of mathematics skills. This article provides a process that teachers can follow easily to gain confidence in their abilities to create and conduct informal mathematics assessments based on their classroom curriculum. (Contains 8 figures.)