This study was intended to investigate teacher’s professional development initiative by an educational foundation in free of cost foundation schools. The appointment of teaching associates is done by the educational foundation funded by Government of Sindh as English language specialists for English language teachers to enhance their knowledge, improvement in teaching practices, learning different language teaching methods and strategies and developing their overall teaching skills. For this purpose data was collected through semi-structured interviews of English language teachers and Teaching Associates to gain knowledge of their perceptions regarding effectiveness of in-service professional development. Observations of language teachers were also conducted to deeply investigate the extent to which Teaching Associates have been successful to achieve objectives set for English language teaching by foundation and the extent to which in-service professional development by Teaching Associates has been implemented in English language classrooms of foundation schools. The data obtained by semi-structured interviews has been analyzed thematically. Whereas, data obtained from observations has been analyzed through SPSS Version 26. The results showed that the in-service professional development has put positive effects on capacity building of teachers. Majority of teachers have been benefited in terms of Instructional planning, proper lecture delivery, Usage of relevant methods and strategies, Error analysis and error correction in language classes along with usage of consolidation and assessment techniques to improve their performance as well as the performance of language learners. The study recommended proper consideration towards usage of English language in the language classes and attention towards conceptual clarity. Thus, TAs need to work more on their planning for professional development of language teachers in foundation schools. Keywords: Professional development, Teacher education, Model classes, Training sessions, class observations, Foundation schools. DOI: 10.7176/RHSS/10-17-03 Publication date: October 31 st 2020