To ensure that all children are on positive developmental trajectories, early education professionals need scientifically-based assessments of children’s growth and development. Such assessments meet established standards for measurement and therefore support valid and reliable inferences about children’s functioning. This study used the Mental Measurement Yearbook to provide a comprehensive examination of the degree to which early childhood assessment developers report evidence of validity and reliability in line with the Standards for Educational and Psychological Testing. The Mental Measurement Yearbook provides the most comprehensive and continuous examination of assessment developers’ development and validation practices. Findings from this study suggest that only a small percentage of the 404 early childhood assessment reviews in the MMY provide evidence on all of the critical sources of validity outlined in the Standards. Response processes and consequences of testing are the most neglected sources of validity evidence. Implications of the findings from this study for early childhood assessment development, validation, and use are discussed. (author abstract)