The present invention provides methods for the use of visible and/or near-infrared spectroscopic methodology to monitor and control processes for the generation of particles, including processes that provide for a reduction in particle size and processes that result in an increase in particle size. One embodiment of the present invention employs visible and/or near-infrared diffuse reflectance spectroscopy to monitor particle size. The present invention is particularly useful for monitoring particle size of optically dense samples and is further useful for monitoring the endpoint of particle generation processes. In contrast to methods known in the art, the present invention is especially useful for on-line monitoring of particle size.