Low access blood flow has been recognized as the most important cause for access thrombosis and subsequent access failure so that some form of access flow surveillance is recommended in everyday practice. The classic technique to measure flow in physiology is based on indicator dilution as most flow rates are inaccessible to direct measurement. However, extracorporeal blood purification techniques have been designed for the controlled removal and/or delivery of solutes, all of which can be used as indicators to measure selected transport characteristics throughout the intra- and extracorporeal system. It is therefore not surprising that extracorporeal techniques are extremely well suited for access flow monitoring methods based on indicator dilution, also because these techniques can be integrated into the extracorporeal system as part of the purification process and as these procedures have the potential to be fully automated. In this chapter the physiological basis of indicator dilution is briefly summarized with regard to application in hemodialysis considering the limitations as well as the possibilities for integration and automation.