Although teachers graduate from education faculties and are equipped with pedagogical content knowledge, throughout their teaching years they face unique circumstances which may require additional training, or over the years they may experience gradual loss of knowledge or skills. That’s why in-service teachers should be in constant professional development in order to deal with such situations. In-service training (INSET) refers to the process of increasing the knowledge and skills of teachers and assessing and developing their professional approach once they receive their certification in teaching and are professionally employed. This paper presents the results of a detailed survey into the historical development of INSET activities for foreign language teachers beginning with the period following the foundation of the Turkish Republic in 1923 till present. In so doing, it also attempts to portray the general picture of motivations behind the trainings and the outcomes obtained from some of these professional development activities. It was seen that although there has been an increase in the number of trainings and training units throughout time, INSET activities signal a need for modifications in engaging teachers in an experiential process facilitating growth, reflection, and change. For effective INSET practices, a social constructivist context is necessary in which individuals make their own meanings through reflection, experience and dialogue. The paper will also present pedagogical suggestions for effective INSET practices targeting foreign language teachers.