Addressing a critical need for effective vocabulary practices in early childhood classrooms, we conducted a design experiment to achieve three goals: (1) developing a comprehensive model for early childhood vocabulary instruction, (2) examining the effectiveness of this model, and (3) discerning the contextual conditions that hinder or facilitate its implementation. Guided by sociocultural theory and the design experiment framework, our study was conducted in one Head Start classroom and included a control class for comparison. The resulting model extends the existing instruction in several ways and effectively increases children's target vocabulary knowledge. Contextual constraints were identified (high teacher turnover, low teacher buy-in, and rigid teacher roles) and addressed by working with the teachers towards developing a community of practice. (author abstract)