Long-term ecological research (LTER) has a strong foundation in the fields of biogeosciences and ecosystem research with infrastructures established in the nineteenth (e.g., 1891—Plon, Germany; 1840—Rothamsted, England) and twentieth centuries (e.g., 1906—Lunz, Austria; 1925—Trout Lake Station, WI, USA; 1955—Hubbard Brook, NH, USA; 1960—Solling, Germany). Over time, it has been repeatedly shown that the long-term study of processes has the potential to significantly improve ecological understanding at the ecosystem or larger scales. The international network of LTER sites (ILTER) was established in 1993 and included 28 countries by the turn of the millennium. The geographical spread of LTER sites across the globe has significantly increased since then, and by 2016, 44 countries have established formal LTER programs with approximately 800 LTER sites and 70 Long-term Socio-ecological Research (LTSER) platforms as part of the international network. Stimulated by the World Summit on Sustainable Development and the UN Convention on Biodiversity, research topics at ILTER sites have diversified considerably. Scientists started to address societal processes driving environmental change, as well as feedbacks of environmental changes on society. Furthermore, biodiversity research became an intrinsic part of LTER in many countries initializing the establishment of research infrastructures in ecosystems not yet included (e.g., caves, marine habitats). In October 2016, the ILTER Open Science Meeting in the Kruger National Park, South Africa, gave an impressive overview of the diversity of LTER across the globe today. With this special issue, we aim to present the broad range of research currently conducted across the ILTER network with a collection of papers from different continents and with research topics both classical to LTER and emerging fields of contemporary LTER.