The transformation of bio-based sidestreams and raw materials into food ingredients represents a crucial step in the promotion of a sustainable circular economy within the framework of a ‘food first’ approach. From this perspective, the utilisation of sidestreams from the food and agricultural industries is intended to optimise the value of residual streams and minimise food loss and waste. By employing a range of processing techniques, including dry fractionation and wet extraction, it is possible to isolate and refine valuable components such as proteins, fibres and bioactive compounds for conversion into safe and high-quality food ingredients. This approach not only contributes to the sustainable utilisation of resources but also offers new avenues for the development of functional foods with enhanced nutritional properties. The chapter begins with an overview of potential sidestreams in the food sector and for the food sector as a whole. It is essential to recognise that only safe food can be subjected to further processing. Consequently, the evaluation of food sidestreams in terms of food safety and nutritional content is addressed in accordance with this premise. This chapter places particular emphasis on the fractionation and extraction procedures, which are of crucial importance in ensuring the quality and accessibility of these ingredients for food applications. Technological innovations also play a pivotal role in optimising the efficiency and cost-effectiveness of transformation processes. Finally, use cases, such as Brewers' spent grain and others, are presented to illustrate the integration and discussion of the acquired knowledge.