Negative early experiences have long-lasting and consequential impacts, leaving children more susceptible to poor health, poor educational performance and even criminal justice involvement over the course of their lives. Recognizing the tremendous opportunities--and risks--associated with this critical period of brain development, policymakers are increasingly investing in what experts call "infant and early childhood mental health" (I-ECMH). The purpose of this brief is to 1) explain what I-ECMH is; 2) document why investments in I-ECMH matter; and 3) identify specific actions that state policymakers should deploy to support the healthy development of young children. (author abstract)