Optical sensors have advanced significantly over the past 2 decades leading to today's noninvasive optical measurement capabilities and their widespread applications in renal therapy. These measurements provide significant advantages to the clinician. For example, a given blood constituent can be monitored in real time (continuously, nondestructively), which facilitates the ability to optimally "titrate" the therapy with immediate visual feedback. Optical methods have another intrinsic advantage in that each biologic constituent has its own unique spectral "signature" allowing for simultaneous, multiple, and specific measurements of biologic analytes. Use of this budding spectral technology in renal therapy today provides for increased patient safety (by measuring plasma-free hemoglobin, microemboli, clots, oxygen saturation, blood leaks, and hematocrit), measurements of dialysis dose (dialysate urea levels), dry weight (tissue water monitoring), access viability (recirculation, access blood flow), cardiac status (absolute blood volume, cardiac output), and enhanced continuous fluid management (fluid overload, critical blood volume). As microelectronics and signal processing capabilities continue to advance, so will the future of optical diagnosis and treatment. These capabilities translate directly to improved patient quality of life.