A wearable infusion pump assembly comprising: a reservoir (118,2918) for receiving an infusible fluid; and a fluid delivery system configured to deliver the infusible fluid from the reservoir (118,2918), to an external infusion set, wherein the fluid delivery system includes: a volume sensor assembly (2920, 2946); a pump plunger (106,2902) for extracting a quantity of infusible fluid from the reservoir and providing the quantity of infusible fluid to the volume sensor assembly (2920, 2946), wherein the volume sensor assembly (2920, 2946) is configured to determine the volume of at least a portion of the quantity of fluid in the volume sensor assembly (2920, 2946); a reservoir valve configured to selectively isolate the pump plunger from the reservoir (118,2918); and a measurement valve configured to selectively isolate the volume sensor assembly from the external infusion set; and a first optical sensor assembly (2956,2958) characterized in that the first optical sensor assembly (2956, 2958) is configured to sense the movement and distance of displacement of the pump plunger; and the fluid delivery system further comprising a second optical sensor assembly wherein the second optical sensor assembly is configured to sense the movement of the measurement valve. a reservoir (118,2918) for receiving an infusible fluid; and a fluid delivery system configured to deliver the infusible fluid from the reservoir (118,2918), to an external infusion set, wherein the fluid delivery system includes: a volume sensor assembly (2920, 2946); a pump plunger (106,2902) for extracting a quantity of infusible fluid from the reservoir and providing the quantity of infusible fluid to the volume sensor assembly (2920, 2946), wherein the volume sensor assembly (2920, 2946) is configured to determine the volume of at least a portion of the quantity of fluid in the volume sensor assembly (2920, 2946); a reservoir valve configured to selectively isolate the pump plunger from the reservoir (118,2918); and a measurement valve configured to selectively isolate the volume sensor assembly from the external infusion set; and a first optical sensor assembly (2956,2958) characterized in that the first optical sensor assembly (2956, 2958) is configured to sense the movement and distance of displacement of the pump plunger; and the fluid delivery system further comprising a second optical sensor assembly wherein the second optical sensor assembly is configured to sense the movement of the measurement valve. a volume sensor assembly (2920, 2946); a pump plunger (106,2902) for extracting a quantity of infusible fluid from the reservoir and providing the quantity of infusible fluid to the volume sensor assembly (2920, 2946), wherein the volume sensor assembly (2920, 2946) is configured to determine the volume of at least a portion of the quantity of fluid in the volume sensor assembly (2920, 2946); a reservoir valve configured to selectively isolate the pump plunger from the reservoir (118,2918); and a measurement valve configured to selectively isolate the volume sensor assembly from the external infusion set; and a first optical sensor assembly (2956,2958) characterized in that the first optical sensor assembly (2956, 2958) is configured to sense the movement and distance of displacement of the pump plunger; and the fluid delivery system further comprising a second optical sensor assembly wherein the second optical sensor assembly is configured to sense the movement of the measurement valve.