There is a scarcity of empirically validated treatments for infants and toddlers under age 3 years with autism spectrum disorders (ASD), as well as a scarcity of empirical investigation into successful intervention characteristics for this population. Yet early screening efforts are focused on identifying autism risk in children under age 3 years. In order to build ASD interventions for infants and toddlers upon a foundation of evidence-based characteristics, the current paper presents the results of a systematic literature search and effect size analysis of efficacious interventions for infants and toddlers with other developmental disorders: those who were born prematurely, have developmental impairments, or are at high risk for developmental impairments due to the presence of a biological or familial condition associated with developmental impairments. A review of 32 controlled, high-quality experimental studies revealed that the most efficacious interventions routinely used a combination of four specific intervention procedures, including (1) parent involvement in intervention, including ongoing parent coaching that focused both on parental responsivity and sensitivity to child cues and on teaching families to provide the infant interventions, (2) individualization to each infant’s developmental profile, (3) focusing on a broad rather than a narrow range of learning targets, and (4) temporal characteristics involving beginning as early as the risk is detected and providing greater intensity and duration of the intervention. These four characteristics of efficacious interventions for infants and toddlers with other developmental challenges likely represent a solid foundation from which researchers and clinicians can build efficacious interventions for infants and toddlers at risk for or affected by ASD.