**Abstract**

This chapter presents origin and physical basis of the optical method for traditional haemodialysis (HD) dose assessment, accepted as a valid bloodless, robust, automatic, *in situ* and online monitoring technology in clinical praxis. Dialysis dose Kt/V, total removed urea (TRU) and the nutrition parameters PCR, nPCR estimation from ultraviolet (UV) absorbance in the spent dialysate is explained. Since urea, a small watersoluble uremic solute and a surrogate marker for the efficiency of dialysis treatment to clear the blood of toxins and metabolic end products, is not representative for all retained uremic toxins removed with the modern dialysis care, new developments of optical online monitoring of uremic toxins, beyond urea, are discussed. Optical intradialytic monitoring of small-, middle- and protein-bound molecules' removal, exemplified by marker molecules uric acid, beta-2 microglobulin and indoxyl sulphate, is described. A new concept and sensor technology for multi-component uremic toxins' intradialytic optical monitoring of spent dialysate with some clinical examples are introduced. Drug interference studies during the optical dialysis monitoring and future directions in optical monitoring are included. Offered benefits will be more patient-centred, integrated and cost-efficient care, as feedback for clinicians helps to improve and personalize the treatment quality, minimizing costly adverse effects.

**Keywords:** dialysis dose, dialysis adequacy, fluorescence spectroscopy, haemodialysis, online monitoring, optics, small uremic toxins, middle molecule uremic toxins, protein-bound uremic toxins, ultraviolet absorbance, solute removal, spent dialysate
