The World Bank created this Early Child Development (ECD) Guide in response to a growing demand from Task Team Leaders (TTLs) for advice and support to facilitate the policy dialogue on the topic of ECD and to help policy makers make and implement relevant choices on how to best invest in ECD in the context of their country's economy and national priorities. This Guide fills a gap in the literature by (i) distilling existing information in a user-friendly format, (ii) providing practical information on topics that have recently become particularly relevant in ECD (e.g., measuring child development outcomes through the identification and adaptation of relevant instruments, conditional cash transfers for families with young children, etc.), and (iii) assessing the quality of the latest evidence on each topic and identifying the knowledge gaps/remaining questions for which additional experimentation and evaluation are required. This Guide is designed as a series of short notes (approximately 6-8 pages each), clustered into thematic sections: (i) Initiating the policy dialogue: Why invest in ECD? (3 notes); (ii) Assessing needs, measuring outcomes, and establishing policy frameworks (2 notes); (iii) "Strategic entry points" for ECD investments (4 notes); and (iv) Costing and financing (2 notes). The notes are summarize the main debates in the field. Each note is designed to be read independently, so information is sometimes repeated or cross-referenced across notes.