This thesis offers a collective portrait of an important group of scientific and technical practitioners in India from 1900 to 1947: professional engineers. It focuses on engineers working in three key sectors: public works, railways and private industry. Based on a range of little-used sources, it charts the evolution of the profession in terms of the composition, training, employment patterns and work culture of its members. The thesis argues that changes in the profession were both caused by and contributed to two important, contested transformations in interwar Indian society: the growth of large-scale private industry (industrialisation), and the increasing proportion of ‘native’ Indians in government services and private firms (Indianisation). Engineers in the public works and railways played a crucial role as officers of the colonial state, as revealed by debates on Indianisation in these sectors. Engineers also enabled the emergence of large industrial enterprises, which in turn impacted the profession. Previously dominated by expatriate government engineers, the profession expanded, was considerably Indianised, and diversified to include industrial experts. Whereas the profession was initially oriented towards the imperial metropolis, a nascent Indian identity emerged in the interwar period. Throughout, the thesis studies British and Indian engineers in parallel. It also underscores the importance of studying the history of science, technology and medicine in twentieth-century India in relation to the heterogeneous, evolving colonial state. Finally, the focus on practitioners complements the existing historiographical emphasis on intellectuals’ debates on science, colonialism, modernity and nation.