This paper presents the Nubase2012 evaluation that contains the recommended values for nuclear and decay properties of nuclides in their ground and excited isomeric (T1/2>=100 ns) states. All nuclides for which some experimental information is known are considered. Nubase2012 covers all up to date experimental data published in primary (journal articles) and secondary (mainly laboratory reports and conference proceedings) references, together with the corresponding bibliographical information. During the development of Nubase2012, the data available in the “Evaluated Nuclear Structure Data File" (Ensdf) database were consulted, and critically assessed of their validity and completeness. Furthermore, a large amount of new and somewhat older experimental results that were missing in Ensdf were compiled, evaluated and included in Nubase2012. The atomic mass values were taken from the “Atomic Mass Evaluation" (Ame2012, second and third parts of the present issue). In cases where no experimental data were available for a particular nuclide, trends in the behavior of specific properties in neighboring nuclei (TNN) were examined. This approach allowed to estimate, whenever possible, values for a range of properties, and are labeled in Nubase2012 as “non-experimental" (flagged “#"). Evaluation procedures and policies that were used during the development of this database are presented, together with a detailed table of recommended values and their uncertainties.