The Race to the Top - Early Learning Challenge (RTT-ELC) grants program, sponsored by the U.S. Departments of Education and Health and Human Services, aimed to improve children's access to high quality early care and education. RTT-ELC awarded more than $1 billion over three rounds of grants to help states develop and implement systems that rate early learning and development programs on quality and help them improve. These systems are known as tiered quality rating and improvement systems (TQRIS). To strengthen the quality of early learning and development programs, TQRIS rate programs on quality standards and publicize the ratings of individual programs. States can use these ratings to identify low quality programs that need to improve, and parents can use the ratings to choose high quality programs for their children. However, the usefulness of the ratings for these purposes depends on how accurately they measure programs' quality, that is, their validity. To inform states' continued development of TQRIS and future validation studies, this report synthesizes findings from validation studies conducted by nine states that received RTT-ELC grants. It also describes the challenges that researchers faced when conducting these studies. [Additional contributions to this report were made by Michael Ponza, Kimberly Boller, Lisa Dragoset, Gretchen Kirby, Hanzhi Zhou, Amanda Lee, Malik Mubeen, John Kennedy, Cindy Castro, and Sharon Clark. For the related reports "Moving on Up? Program Quality Ratings under Race to the Top - Early Learning Challenge. Evaluation Brief. NCEE 2019-4000," see ED594514; "Quality Ratings and System Characteristics: Patterns in the Round 1 Race to the Top - Early Learning Challenge States. Evaluation Brief. NCEE 2019-4004," see ED594512; and "Implementation of Tiered Quality Rating and Improvement Systems in States That Received Race to the Top - Early Learning Challenge Grants. Study Highlights," see ED594516.]