In response to recently identified research priorities by TESOL (Teachers of English to Speakers of Other Languages) and AERA, the objective of this documentary account is to describe and evaluate a professional development project for in-service teachers working with diverse English Language Learners (ELLs). The purpose of our project was to merge two distinct professional development models for teachers who educate ELLs without prior training or certification. The “lesson study” approach, which began in Japan as a professional development movement was adapted and combined with the SIOP (Sheltered Instruction Observation Protocol) Model. Findings from a combination of quantitative and qualitative data sources indicated that the lesson study approach merged with the SIOP may warrant systematic implementation in in-service teacher education.