Human organ-on-a-chip (OOC) is an emerging technology that incorporates human adult cells in a small laboratory device to mimic how organs work—such as the brain, heart, or lungs. OOCs may be designed to simulate the mechanics of human organs, such as a lung-on-a-chip that stretches to simulate breathing. Researchers are developing and using OOCs to model diseases and predict responses to chemicals. For example, companies are using OOCs to assess aspects of drug safety and efficacy. Because OOCs contain human cells, their use in research may have more relevance to humans than animal testing or other conventional lab methods, but certain OOC research may cost more and take longer. Currently, OOCs cannot replace animal testing but may be used alongside animals. The next generation of OOC is focused on developing “body-on-a-chip” systems that link together multiple OOCs—such as the intestine, liver, and kidney—to investigate how organs interact.