The production and use of slow recovery “viscoelastic” polyurethane foams have increased rapidly over the past few years. Their unique performance characteristics, which include shape conformance, vibration and sound damping, and energy and shock absorption, make these the foams of choice for many diverse applications such as pillows, mattress toppers, automotive trim, sports and medical equipment, and ergonomic cushions and pads. These diverse applications require a wide range of foam grades with tailored performance properties. New polyol and formulating technology is being developed to meet the increased demands for improved processing, wider grade latitude and improved performance properties of viscoelastic foam. This paper reports the results of systematic experimental design studies that were conducted in the laboratory to help develop this technology. A key advance is the development of polyol compositions that enable the production of high quality, soft viscoelastic foams at 100 index and above. This eliminates processing inconsistencies associated with low index foaming, and yields foams with superior performance characteristics such as improved compression sets, reduced discoloration, and finer cell structure with improved feel. The design studies also identified formulating routes to optimize key foam performance features such as low resilience, slow but complete recovery, maintenance of slow recovery characteristics with use, reduced temperature sensitivity, good airflow, and good strength properties. The results of machine foam studies will be presented along with additional foam performance results.