The aim of this study was to determine preschool teachers' STEM training, STEM classroom activities, lesson planning processes, problems, and evaluation strategies and methods during activities. The study sample consisted of 20 preschool teachers. Participants were recruited using criterion sampling, which is a purposive sampling method. This was a case study, which is a qualitative research method. Data were collected using a semi-structured interview form. Qualitative data were analyzed inductive content analysis. Themes, categories, and codes were developed according to the content analysis results in line with the research purpose. Content analysis results showed that participants implemented different preschool STEM activities and used different strategies, methods, and techniques while implementing those activities. They also stated that they faced various problems during those activities. They stated that the STEM training helped them to develop professional competence. However, they had difficulty planning lessons in line with STEM education, due to lack of content knowledge.