Current knowledge of the nuclear physics of fuels and materials provides an understanding and simulation of the operations of nuclear reactors and other systems, both under ordinary and exceptional circumstances. As part of a broad spectrum of collaborative activities underpinning research in basic nuclear sciences, the Nuclear Energy Agency (NEA) is supporting collaboration between experimentalists, theoreticians and modelling experts to advance the state of the art in nuclear data. This report offers an overview of collective results from 31 institutions in 15 NEA member countries, along with results from technical experts in the People's Republic of China, in the context of the NEA Collaborative International Evaluated Library Organisation (CIELO) Pilot Project. It reviews recent developments resulting from new measurements and semi-empirical models, as well as the validation of the CIELO nuclear data evaluations against suites of systems representing a wide range of current and future nuclear facilities. The CIELO project has delivered new, evaluated data for the isotopes of uranium, plutonium, iron, oxygen and hydrogen, which have been adopted in all nuclear data libraries released since the CIELO project was completed.