Inclusion Criteria:  Aged 18-80. Maintenance dialysis for ESRD for at least 6 months. Dialysis via an AVF or AVG in an arm location. Ability to understand and sign a written informed consent form, which must be obtained prior to initiation of any study procedures. Can speak, read and communicate in English. Willing and able to wear the SmartPatch as instructed and comply with the visits, tests, and schedule of evaluations.  Exclusion Criteria:  Hospital admission within last 30 days which, in the opinion of the physician or study staff, could potentially affect the subject's ability to participate in the study procedures. Pseudoaneurysm on the measured AV access. Liver disease associated with serum albumin levels less than 35 g/L. History of acute illness episode that, in the opinion of their physician or study staff, could potentially affect study data being collected. Significant fluid overload (patient > 3kg above dialysis dry weight at start of dialysis session) Subjects with novel materials for their AVF or hAVG, such as HeRO grafts, chest-wall AVFs Subjects with rapid-use grafts (e.g., AVflo™ or Gore® Acuseal) Participation in any other clinical trials involving investigational or marketed products within 30 days prior to entry in the study if, in the judgement of the study staff, participation in this study could potentially affect the quality of study data. Other conditions that in the opinion of the Principal Investigator may increase risk to the subject and/or compromise the quality of the clinical trial. The Study Doctor(s) determine(s) that the subject is not eligible for participation in this research study.