Abstract: Embodiments herein relate to water in fuel sensing systems that can be mounted on-vehicle. In an embodiment, a water in fuel sensing system is included having a light source, a light detector, and a sensor controller, wherein the sensor controller is in signal communication with the light detector and the sensor controller is configured to evaluate signals received from the light detector, identify water droplets based on the signals received from the light detector, record information regarding the classified water droplets, and generate an estimate of an amount of water in a fuel. Other embodiments are also included herein.