The Child Observation Record (COR), initially developed in 1993 by HighScope Educational Research Foundation, is an observation-based instrument that provides systematic assessment of young children's knowledge and abilities in all major areas of development. Teachers or caregivers spend a few minutes each day writing brief notes or "anecdotes" that objectively describe significant episodes of young children's activities. The anecdotes are then classified and scored according to various COR categories, items, and levels, providing a comprehensive portrait of each child's developmental gains and the progress of the group as a whole. In 2012, the COR was revised to span the developmental range from infancy and toddlerhood through kindergarten. The validation evidence for the revised instrument, the COR Advantage, was gathered in two phases--an initial validation phase and the multi-state implementation validation phase. The results from both phases show that the COR Advantage generates scores which are internally consistent and inferences made from scores that are valid for (a) documenting the developmental trajectories of children from birth through kindergarten in all key areas of children's development, and (b) capturing developmental milestones of children from varied backgrounds with diverse abilities when used by early childhood professionals.