The preservice and in-service predictors of 1,668 Part C early intervention and Part B(619) preschool special practioners' perceived self-efficacy beliefs are reported. The preservice variables were type of degree (discipline), years of formal postsecondary education, licensure, and participants' judgment of how well their preservice training prepared the practioners to work with young children and their families. The in-service variables were type of state training/technical assistance available to the participants, whether participants were required to have continuing education, and the amount of in-service training the participants received. Self-efficacy was measured in terms of the participants' perceived confidence and competence to successfully enact three procedural practices (family-centered practices, teaming practices, and assessment in education practices), and three intervention practices (IFSPs/IEPs, instructional practices, and natural environment/inclusion practices). Results suggest that participants' judgments of their preservice preparedness to work with young children and their families, and the number of different kinds of in-services training for which they availed themselves, were the best predictors of all four types of self-efficacy beliefs. Implications for preservice and in-service professional preparation and training are discussed. (author abstract)