1) to evaluate an access flow threshold for access flow monitoring in regard to detection of venous outflow stenosis, (2) to confirm the relationship between AV access flow and the likelihood of AV access thrombosis, and (3) to define the impact of intervention with both percutaneous transluminal angioplasty (PTA) and surgery on access flow, and ultimately access thrombosis on of AV fistulae and AV grafts.