Far infrared (FIR) textiles are a new category of functional textiles that have putative health and wellbeing functionality. FIR exerts strong rotational and vibrational effects at the molecular level with the potential to be biologically beneficial. The underpinning targeted benefit of FIR textiles is the enhancement of blood circulation. These materials are based on the principle of absorbing energy from sunlight and then radiating this energy back onto the body at specific wavelengths. This chapter explores the basic principles of electromagnetic radiation as they relate to FIR, examines the putative mechanism and therapeutic effects of FIR therapy, and surveys what objective evidence currently exists of health benefits. The role of FIR fibres and fabrics in functional textiles is also evaluated, together with existing and potential applications of these textile products. A summary of the benefits, limitations and anticipated future trends of current products and technologies is then provided.