It is almost 20 years since NNRTIs were identified as a new class of antiretroviral drugs for the treatment of HIV-1 infection. Although they belong to different and diverse chemical families, they share a common and unique mechanism of action: their interaction with HIV-1 reverse transcriptase induces conformational changes that inhibit the catalytic activities of the enzyme. They are characterized by their specificity for HIV-1, which makes them very selective inhibitors of the virus. First generation NNRTIs nevirapine and efavirenz, in combination with other antiretroviral drugs, have become a cornerstone for the treatment of HIV-1 infection, in patients initiating antiretroviral therapy. Further research has led to the discovery and development of next generation NNRTIs with an increased genetic barrier to the development of resistance. Etravirine is the first NNRTI to show sustained virologic efficacy in patients with NNRTI resistant HIV-1. This review covers the NNRTI class of anti-HIV-1 drugs, from the initial discovery of the class in 1990 to the current compounds in clinical development, i.e. around 20 years of research and development efforts. It describes the characteristics of the NNRTIs, their mechanisms of action, HIV-1 resistance to the inhibitors, and the drugs that have been approved for the treatment of HIV-1 infection, or are currently in clinical development. The role of NNRTIs in prevention of HIV transmission is also addressed. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, vol. 85, issue 1, 2010