There is described an apparatus for monitoring particles in a fluid stream, comprising a body portion and a detector element that is acoustically decoupled from the body portion. The detector element comprises a target surface, a sample acoustic sensor and a corrosion sensor. The sample acoustic sensor is acoustically coupled to the target surface and is arranged to provide a first signal, which varies in dependence upon acoustic noise generated by impacts of particles and fluid on the target surface. The corrosion sensor is arranged to provide a second signal, which varies in dependence upon corrosion and/or erosion of the target surface. A corresponding method of monitoring particles in a fluid stream is also described. The method and apparatus are suitable for monitoring sand in oil and gas production flow streams.