This chapter introduces the book by examining the critical role of functional integration in both the life sciences and philosophical discourse. In life sciences—encompassing cell biology, evolutionary biology, physiology, and pathophysiology—functional integration describes the causal interconnections among an organism’s components (such as cells, tissues, and organs) that enable systemic functioning. Philosophically, it is central to debates on biological functions, where it underscores the systemic nature of functions, and in discussions about mechanisms, it highlights the interplay between different mechanisms that drive biological activities. Within the debate on biological individuality, functional integration primarily elucidates the physiological aspects of individuality, although its implications for evolutionary dimensions remain less explored. This chapter argues that the interpretation of functional integration varies across contexts, highlighting its multifaceted significance in understanding the complexity of life and individuality.