Humans serve as hosts for a number of parasitic roundworms. As a group, these parasites reproduce sexually with adult females releasing up to 100,000 eggs each day. Fertilized eggs hatch into larvae, which grow into adults. When the adult forms reach sexual maturity, the cycle begins again. Many infected individuals are completely asymptomatic or only mildly symptomatic. Infected children may experience nutritional deficiencies. Symptomatic individuals may experience fever, fatigue, rash, abdominal discomfort, vomiting, nausea, or diarrhea. High worm burden localized to the intestine can lead to serious illness such as small bowel obstruction, acute appendicitis, pancreatitis, or hepatobiliary disease. Extraintestinal disease may affect the lungs, liver, spleen, brain, eye, and/or other organs. Here, we summarize the life cycles, geographic distribution, and defining characteristics for each of the specific parasitic nematodes known to infect humans.