Precision Viticulture is a relatively new management method which tailors vineyard inputs according to site specific conditions. This thesis describes a grape harvester yield monitoring system that was developed to measure the instantaneous and total weight of grape material as it flowed through a grape harvester. An in-lab test-bed was developed to provide dynamic calibration and analyze the yield monitor prototype in a controlled and repeatable environment. In-lab testing resulted in an average error in measurement of -2.4% between actual and yield monitor weights. Field testing indicated an average error of 7.1% between total harvested weight and yield monitor weight. Results showed that the method used to measure the mass flow rate of grape material is capable of meeting the demanding requirements of a precision yield monitoring system.