Onunaku, Ahlers, and Gilkerson describe Illinois's effort to build infant mental health capacity within the Part C Early Intervention system and Wisconsin's effort to build capacity for infant and early childhood mental health services statewide across all systems that serve children. Because of multiple funding streams, families often experience fragmented services that fail to meet their children's needs. Efforts to build early childhood mental health systems include weaving infant and early childhood mental health services into existing early childhood systems, such as early care and education, pediatric health care, and child welfare. Comprehensive systems include professional development and training; a full range of mental health services; effective policies; public awareness; and systems collaboration. The authors describe national effort to develop a network of communities and states across the country that can provide support and technical assistance to each other in developing a system of infant and early childhood mental health.