Indirect, rapid, and useful method of measuring microbial growth, Turbidity proportional to cell number (within limits), Most often measured with a spectrophotometer and measurement referred to as optical density (O.D.), Quick and easy to perform, Typically do not require destruction or significant disturbance of sample, Sometimes problematic (e.g., microbes that form clumps or grow as biofilms), Not accurate at high densities, Doesn't show if culture mixed, Doesn't differentiate between live and dead cells