In this paper we have tried to document some of the recent advances in the conceptualization and assessment of early-emerging social-emotional and behavior problems, competencies, and psychopathology. Considerable evidence documents that young children evidence significant psychopathology (cf., Del Carmen & Carter, in press; Emde, 1999; Zeanah, 2001; Zeanah et al., 1997). Given the range of new assessment measures that have become available over the past 10 years, the field of young child mental health is poised for dramatic gains in knowledge. It is critical to conduct large-scale, longitudinal, epidemiological studies to inform our understanding of the course of psychopathological conditions within the context of a normative developmental framework. Multi-method, multi-informant assessment approaches are more essential in early childhood due to young children's inability to provide self-reports and the embedded nature of children's development in their caregiving contexts. Screening large representative samples affords the opportunity to ascertain unbiased clinically informative sub-samples for methodologically intensive sub-studies. These sub-studies can address the child's cognitive and linguistic developmental capacities as well as utilize observational methods to examine the relational context. This approach provides an opportunity to merge dimensional and diagnostic assessments and will yield critical information for disentangling continuities and discontinuities in normative and atypical development. The assessment methodology currently exists to routinely screen very young children for social-emotional and behavior problems as well as delays in the acquisition of competencies in pediatric settings as well as in early intervention programs. Yet, despite the likely long-term benefits and cost-saving potential of early identification and intervention services, short-term cost and knowledge barriers currently limit widespread implementation. Discussions with pediatricians suggest that one of the greatest barriers to screening is the limited availability of mental health referral sources. Indeed, very few children who are rated by parents as having elevated social-emotional and behavior problems are receiving any behavioral health services (Horwitz et al., in press). Unmet mental health needs exist among non-referred children in the community as well as among children receiving early intervention services for developmental concerns. Documenting the mental health needs of young children may promote training of professionals who have the competence to treat young children and their families. Moreover, the availability of social-emotional and behavior problem assessment tools should increase studies that focus on the clinical efficacy and effectiveness of prevention and early intervention programs designed to promote positive mental health. Finally, although significant progress is occurring in the arena of young child diagnosis, a strong case can be made for intervening when young children are exhibiting elevations in problem behaviors or delays in the acquisition of competence. This is particularly true when children are also experiencing exposure to multiple contextual risk factors. It is therefore important to advocate for changes to systems that require child diagnosis as a gateway to intervention. As we learn more about the precursors or prodromal manifestations of clinical psychopathology we will be able to examine the efficacy of earlier targeted preventive intervention approaches.