Abstract This chapter discusses phosphate-based glasses as biodegradable substitutes with a specific and controllable bioactivity. It starts with a general description of these glasses, highlighting the glass structure theories, formation, chemistry, terminology and properties. It also explores what these glasses can offer in terms of biomedical applications as bone tissue substitutes and antibacterial devices, with a particular focus placed on the in vitro and in vivo studies conducted on both monoliths and glass fibres. The application of these glasses as reinforcing agents for various composites is also considered. The chapter concludes with a discussion of the future of these glasses as biomaterials and highlights possible avenues of potential application.