While increased attention has been put on approaches to teaching science, technology, engineering, and mathematics (STEM), little has been reported on teachers' views, perceptions, and beliefs about teaching STEM. Such knowledge is needed to inform efforts to support innovative teaching of STEM subjects in classrooms. In this study, 154 in-service teachers of both STEM-related and non-STEM related subjects, were randomly selected from schools all over Thailand. Data on teachers' perceptions were captured through a three-part questionnaire including an open-ended question that asked for general background information and perceptions of STEM education and STEM integration. Descriptive statistics were used to analyzed data and content analysis was used to analyze open-ended responses. Research findings showed that the majority of teachers (85.5%) had never heard about STEM education and approximately 19% of in-service teachers could not provide a definition of STEM education, with 20.53% of the teachers viewing STEM as a transdisciplinary course or program. While teachers think that a STEM teaching approach is very interesting, both STEM and non-STEM in-service teachers have strong concerns about the engineering discipline within STEM disciplines.