Wearable blood processing devices offer an attractive solution to problems inherent in clinic-based, intermittent end-stage renal disease therapies. What is involved in transitioning even a part of the current clinic-based population to ambulatory therapy has not been clearly enumerated. This paper addresses what a first-generation wearable device might accomplish, how issues of safety will need to be addressed, and what will make the device attractive to, and manageable by, the patient. Medical, technological, and economic issues are identified.