The aim of this research was to analyze the opinions of teachers working in High Schools on informal learning regarding if they had any informal learning in school environment, which informal learning activities took place, and if this learning had a contribution to their professional development. The research was a qualitative study with a multiple holistic case study design. Data were collected from interviews using semi-structured interview forms. At the end of the research, conclusions were classified under three main themes. The first main theme was that there were some informal learning activities which the teachers were engaged in. The second main theme was that most of the informal learning happened through talking and sharing resources and ideas with others. Especially teacher candidates experienced informal learning through observations of the practices of senior colleagues. The final theme indicated that the informal learning in school environment had a positive effect on teachers' professional development.