Piezoelectric and triboelectric nanogenerators are the two recently developed technologies for effective harvesting of ambient mechanical energy for self-powered systems. The work herein briefly reports the trends and impacts of research based on piezoelectric and triboelectric nanogenerators. For the first time, a statistical study of publication trends in piezo- and triboelectric nanogenerators is used to investigate the current state of the research. We investigate the status of the research in these fields, based on their S-shaped history, as a description of their growth in size over time, considering the number of citations and published papers that use these two novel technologies. A brief discussion of the outlook for these technologies and their potential impact on everyday life is presented, particularly in the development of future 'smart' cities. (C) 2018 Elsevier Ltd. All rights reserved.