Sustainable Development Goal 4 (SDG 4) aims to "ensure inclusive and equitable quality education and promote lifelong learning opportunities for all". In relation to Early Childhood Education (ECE), SDG Target 4.2 aims to "ensure that all girls and boys have access to quality early childhood development, care and pre-primary education so that they are ready for primary education". To operationalise this goal, the primary Indicator 4.2.1 sets out to measure the "proportion of children aged 24-59 months who are developmentally on track in health, learning and psychosocial well-being, by sex". The United Nations Children's Fund (UNICEF) is the custodian of SDG Target 4.2. As such, UNICEF has coordinated efforts to develop an assessment that "captures the achievement of key developmental milestones of children between the ages of 24 and 59 months". This assessment is called the Early Childhood Development Index 2030, or ECDI2030. The ECDI2030 has been in use since March 2020. While the tool provides a quick snapshot for reporting, there is room for improvement, considering the breadth of skills that are developing as children grow. This review of existing ECE assessments has been motivated by the aim of the Global Education Monitoring (GEM) Centre at the Australian Council for Educational Research (ACER) to develop appropriate definitions and to contribute to the objective measurement of skills and attributes in early childhood education. The report reviews a range of assessments, including ECDI2030, with a view to identifying how they can be strengthened. The report is intended to benefit global stakeholders in the SDG 4 agenda, national governments, practitioners, and ultimately, children.