The Kirby-Bauer assay, also called the disc diffusion assay, is a standard procedure used in clinical laboratories to test the susceptibility of patients' bacterial isolates to antibiotics. In the assay, the bacteria are swabbed onto an agar plate, and paper discs impregnated with antibiotics are placed on the agar. The antibiotic diffuses from the disc into the agar, forming a concentration gradient. If the organism is killed or its growth is inhibited, no bacteria will be found in the immediate area around the disc. Using this modification of the assay, the authors aspire that this activity will enable students to learn about the scientific process and gain critical thinking skills through disc diffusion assays. (Contains 4 figures.)