High-resolution rainfall observations are desirable, especially in urban areas. However, most traditional rain sensors measure rain indirectly at a considerable elevation (weather radar), or only in rural areas (WMO standard rain gauges). Opportunistic sensors are devices that are not intended for large-scale rainfall monitoring, but can be used as such. One example is commercial microwave links: microwave signals are transmitted between antennas for the purpose of telecommunication, and the signal power drop due to rainfall can be used to estimate link path-averaged rain rate. Another technique is crowdsourcing of rainfall measurements by amateur weather stations from online weather platforms. These two techniques provide rainfall information based on the already existing infrastructure. Quality control and rainfall retrieval methods are applied to obtain valuable rainfall information. With validation studies the relative potential for operational rainfall monitoring is demonstrated.