Head Start, the largest federally funded early childhood education program in the United States, provides comprehensive services to low-income children and their families. These services historically have a whole child approach, fostering social-emotional well-being, physical and mental health, and cognitive and language development, as well as parent involvement and family social services. In recent years, a number of non-experimental studies have outlined the risk to social-emotional development in young children growing up in poverty. Because children in poverty are exposed to a wide range of psychological and social stressors, they have been found to be at a greater risk for developing emotional and behavioral difficulties compared with their more affluent peers. Low-income children are particularly vulnerable to behavior and emotional difficulties in preschool (Gilliam, 2007), and teachers have reported that they do not know how to address behavioral challenges (Lloyd & Bangser, 2009; La Paro & Pianta, 2000). Head Start has responded to the pressing need for effective tools to strengthen children's social-emotional skills and recent research has shown that well-designed professional development which includes training and coaching can enhance teachers' skills, and strengthen children's social and emotional outcomes (Hemmeter & Fox, 2009; Lloyd & Bangser, 2009; Morris et al, 2010; Raver et al., 2009). Head Start CARES examined enhanced curriculum improvements and professional development in the context of a large-scale random assignment study. The demonstration tested the effects of three theoretically distinct social-emotional program enhancements in Head Start settings across the country in order to determine whether it was possible to effectively implement these programs in a large number of Head Start centers and if so, how. The three structured program enhancements focused on different strategies including training teachers on delivery of classroom management procedures, enhancing children's skills to understand and respond to emotions, and a set of play-based activities designed to support self-regulation. In addition to testing classroom-based strategies, Head Start CARES served as a test of an overall system that was meant to support large-scale implementation of program enhancements in the classroom. This paper focuses on preliminary lessons learned from implementing social-emotional programs supported by a professional development model across the country in varied contexts; in particular the focus is on the training and coaching of teachers in the Head Start CARES demonstration. The seventeen Head Start delegate agencies that were selected to participate in the Head Start CARES demonstration were located in ten states across the nation. In total 307 classrooms and over 3,600 children participated in the study, with one-half receiving the CARES intervention. Training and coaching as a form of professional development offers the opportunity to greatly influence the quality of experiences that both teachers and children in early childhood education settings receive. Implementation of strong coaching and training, however, can be a complicated endeavor, due to the large number of people involved and the varied organizational contexts across grantees. In summary, implementation and scale-up of training, coaching, and the social-emotional enhancements required substantial effort among all of the stakeholders involved. One table and one figure are appended.