The measurement and prediction of photovoltaic energy production is a fundamental sub-discipline in PV system science and engineering. Measurement and monitoring, as compared to prediction, is generally simpler. Prediction, because it is dependent upon weather, is more challenging. Measurement and prediction of PV can be used to compute revenues based upon electricity tariffs, or to create construction contract terms for bonuses or liquidated damages, for utility grid management, or for system owners' warranty and O&M, or many other applications. Whatever the goal, the processes and methods are critical to the physical and financial viability of PV technology and its integration into the utility grid. Though they are described separately in this application note, prediction and monitoring are frequently done simultaneously. This results in a performance ratio. This ratio is a metric which quantifies the expected vs. delivered system production. This application note provides a broad overview and introduction to the topic.