It is a common practice to determine microbial counts for both liquid and solid specimens—suspensions of E. coli in nutrient broth all the way to soil samples and hamburger meat. Most specimens have high enough numbers of microorganisms that the specimen has to be serially diluted to quantitate effectively. This protocol is a step-by-step procedure to working dilution problems, and includes some practice problems at the end. The purpose can be determination of bacterial, fungal or viral counts. This protocol is specific for bacterial counts (colony-forming units, CFUs), but can be modified for fungi (CFUs) and viruses (plaque-forming units, PFUs for viral counts).