In response to the challenges of the 21st century and to the need for students in the public school system to have a higher level of English proficiency, English was declared an issue of national interest in Costa Rica in 2008. For this reason, a decree called Multilingual Costa Rica was signed by the government, setting the stage for an in-service EFL teacher training project. This article deals with the analysis of the design and the implementation of a training model, which led to the improvement of the linguistic skills of approximately 80% of the teacher population in the Costa Rican public school system. This study includes a description of the components of the teacher training model as well as the results of the linguistic performance of the participating EFL teachers on the TOEIC[superscript C] test administered prior to and after three years of training.