This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book discusses the daily struggles and achievements of ‘gender experts’ in the environment and development world. It explores the possible tensions and ruptures that are involved in bringing the feminist knowledges that underpin gender mainstreaming into the realm of technical and scientific knowledges that inform the work of environment and technical development organisations. The book presents a collection of polyvocal essays written together with ‘gender experts’ working in technical and environmental science-led organisations, whose work is for the most part geographically located in the global South. The ‘doing’ of gender mainstreaming or gender-inclusive work has led to the emergence of the ‘gender expert’, a figure that is central to the professionalisation of ‘gender’ within the realm of environment and development organisations, programmes and interventions.