This study draws on the application of sociocultural theory to second-language learning and teaching to examine the impact of a design-based research approach on teacher development and literacy instruction to English-language learners (ELLs). Design-based research methodology was employed to derive theoretical suppositions relating to the process of learning as well as the means by which this process is supported. Our research questions were: (a) How will this professional development model result in shifts in teacher thinking about language and literacy learning for ELLs; (b) what innovations in teachers' repertoires of practice will be developed; and (c) in what ways will these shifts in teachers' thinking and innovations in their repertoire of practice bring about new forms of language and literacy learning? Our findings point to the need to place "development" in the forefront of teacher professional development models. Also foregrounded is the importance of promoting teachers' critical reflection on classroom practices and of creating hypotheses for pedagogical change vis-à-vis new understanding about students' linguistic, cognitive and academic needs.