There are numerous methods of characterizing the particle size distribution of a finely divided solid. Each of these methods is based on a different dimension or characteristic of individual particles and means of depicting the particle population. Each of the methods covers a different range of particle size and is based on a different principle of physics. Identifying a method that is relevant to the application and adopting complementary methods to confirm the observation give greater insight into the particle system being evaluated. Generally, these methods can be divided into direct observation, optical methods based on particle populations, and techniques based on particle motion in a medium.