This paper presents the first version of the NUBASE evaluation of nuclear and decay properties of nuclides in their ground- and isomeric-states. All nuclides for which some experimental information is known are considered. NUBASE has basically been derived from the “Evaluated Nuclear Structure Data Files” and from the “Atomic Mass Evaluation” of 1995, but it also includes information from recent literature or literature missed in one of these two evaluations and is meant to cover all experimental data along with their references. In case no experimental data is available, whenever possible, trends in the systematics of neighboring nuclides have been used to derive estimated values (labeled in the database as non-experimental). Adopted procedures and policies are presented.