Soiling is the process whereby dirt, dust, and organic/inorganic contaminants deposit on the surface of a photovoltaic (PV) module. It causes significant economic losses and can have a substantial impact on the expansion of photovoltaic technologies for energy generation. The first step to address soiling adequately is monitoring, as soiling mitigation has to be tailored to the specific conditions of each PV system and no universally valid strategy exists. The main focus of this study is to assess the current state of the art in soiling monitoring, in order to help the community better understand the needs and the challenges in this area. The potentials and the limitations of each monitoring method are discussed thoroughly in the paper, with the support of original experimental data. An estimation of the future soiling monitoring market trends is also presented, with a forecasted need for tens of thousands of new soiling monitors every year.