There are 1.3 million children in South Africa aged 4-5 years. Approximately 72% (±930,000) attend some kind of Early Learning Programme (ELP). The Thrive by Five Index 2021 is the first (baseline) in a series of surveys that will monitor trends over time in the proportion of 4- to 5-year-old children attending ELPs who are On Track for their age in key areas of development. This is the largest survey of preschool child outcomes ever attempted in South Africa. The Index was initiated by First National Bank and Innovation Edge, in collaboration with the Department of Basic Education (DBE) and supported by the United States Agency for International Development (USAID) and ECD Measure. The Thrive by Five Index provides population-level data on how well preschool children in South Africa (aged 50- 59 months) are doing in three key developmental domains which are known to be associated with longer-term outcomes: Early Learning, Physical Growth and Social Emotional Functioning.