This paper will examine the issues of competence development for teachers' professional work from two points of view towards teachers: first, as teachers and secondly, as professionals in a professional field. Therefore, the paper will question the architecture of the competence development system for teacher professional development in continuing education. As a point of departure in this paper, a discussion about a competence phenomenon will argue the changing approach to teachers' continuing education; competence development will be considered with reference to project work. Teacher's competence development has become a highly complex field because not only professional development in educational field but also competence development in a professional field or subject matter will be considered. Taking this approach into account, teacher's competence development will be analyzed in the context of school education and vocational education. This paper will limit the analysis of teacher's competence development as non-formal (informal) learning activities; therefore, the paper will question accreditation and assessment of competence, which is being developed in a project work. Therefore, project activities as a general framework will be investigated as a background for competence development. Project work will be considered as a non-formal learning environment in continuing education.Discussion of the above-mentioned general theoretical background will be elaborated analyzing the national projects (in Lithuania) of teacher training in non-formal learning environment as well as in professional activities in a vocational field or subject matter. The discrepancies in teacher's competence development for school education teachers and vocational education teachers will be discussed since the national system of teacher's continuing education remains diverse even though the accreditation system of acquired competence is seeking unification either for school education teachers or vocational education teachers. The part of research results will present the scenarios of formalizing the competence development in project activities, because, as preliminary results have shown, teachers regard project work as the best learning experience and suggest assessing the results of this learning in the formal framework of accreditation. Therefore, the research results will inform and support the development of competence measuring instruments in the project work for school education teachers and vocational education teachers. This development will also look for some commonalities in spite of the probability that the system of continuing teachers' education may differ according to school education and vocational education division. The best practice in international partnership projects (school level projects) regarding teachers' competence development in a non-formal learning environment will be considered as a comparative source in order to assess project work as a method competence development.