Textile functionalization represents a transformative advancement in chemistry and textile engineering that surpasses the inherent limitations of conventional textiles. By incorporating advanced physical, chemical, and biological treatments along with smart material systems, textiles are designed to actively respond to environmental stimuli, enhance user performance, and provide multifunctional protection. This functional evolution of textiles enables the development of textiles with integrated properties such as flame retardancy, radiation protection, microbial protection, antistatic properties and electrical conductivity. The advancement of textile functionalization relies on interdisciplinary approaches at the crossroads of theory and principles of surface and bulk modification, as well as hybrid techniques. This chapter begins by defining textile functionalization, its importance, and the physicochemical basis of functionalization, and then proceeds to a discussion of the theoretical foundations and key principles. It presents an overview of the theory and principles of textile functionalization. It concludes by describing the critical role of these theoretical foundations in guiding the design and development of functional textiles and highlighting the potential of textile functionalization to revolutionize their applications across diverse industries.