This thesis aims to explore Indonesian secondary school teachers' knowledge and perceptions of Generative AI (GenAI) in education. Using mixed-method approaches, this study involved 35 secondary teachers in one particular province in Indonesia to fill out an online survey, and four of the participants of this group proceeded to semi-structured interviews. This study employed the Technological Acceptance Model (TAM) as a theoretical framework. The findings indicate that most teachers are familiar with GenAI, with ChatGPT as the prominent tool they use to support the teaching practice. This study also identifies teachers' perception of GenAI benefits in education, including helping them save time for repetitive tasks, generating new ideas for learning content and assisting them as a discussion partner for lesson planning. It also highlights teachers' concerns related to GenAI, involving its reliability, overreliance, privacy and data safety, and its effect on students' learning process. While this study highlights the potential of GenAI in educational practices, it also emphasized several challenges that teachers need to consider when deciding whether to adopt GenAI tools.