This chapter reviews the implementation of Generative Artificial Intelligence (GenAI) in K-12 education, examining its current state and how different education systems are approaching its integration into curriculum frameworks and teaching practices. Through a systematic desk study methodology, we analysed 59 peer-reviewed journal articles, 43 conference proceedings, and 14 policy documents published between 2020 and 2024. Our findings reveal three significant themes in current GenAI implementation. First, in teaching and learning applications, research demonstrates consistent positive impacts of GenAI tools across multiple domains, particularly in language learning and mathematics education, where intelligent tutoring systems and conversational agents have enhanced student learning outcomes. Second, in assessment practices, studies indicate that GenAI systems are transforming educational evaluation through real-time, personalised feedback capabilities. However, significant challenges remain in ensuring learner control and addressing equity concerns. Third, the global analysis of curriculum integration reveals diverse implementation models, from discrete approaches treating AI as a standalone subject to embedded models integrating AI within existing curricula. While there is limited formal curriculum implementation globally, our review identifies significant grassroots development of AI-based educational materials and innovative pedagogical approaches at the school level. Our findings emphasise the importance of contextual factors in successful implementation, including comprehensive teacher preparation, systematic curriculum development, and collaborative design approaches. This study contributes to the broader understanding of effective GenAI curriculum integration strategies while highlighting critical areas requiring further research and development in K-12 education.