In the domain of "Counting & Cardinality," Common Core State Standards indicate that by the time students leave kindergarten, they should know the names and sequence of number words up to 100, be able to count to determine the number of objects in a small set (up to 20), and make comparisons between small sets of objects (National Governors Association Center for Best Practices & Council of Chief State School Officers, 2010). This brief describes the key concepts and skills that are involved in meeting the standards in this domain and provide excerpts from learning trajectories that illustrate how they develop over time and with experience.