Volume I/25A is the first one in a series of volumes on the properties of the excited states of all nuclei. The data presented are collected from all kinds of nuclear reactions, including measurements of reactions with neutrons and γ-rays not yet fully considered in previous compilations. The nuclei considered also comprise nuclei far from the stability line. The properties of excited nuclear states are of importance for scientific and practical applications. No systematic compilation of such data has been performed so far. The present compilation has been prepared by eminent experts in the field. One of the characteristics of Landolt-Börnstein is that data are evaluated before they are accepted for compilation. The idea is to present ‘best values’ which can be used with confidence by non-experts. The present Volume I/25A is providing new data (energy levels, branching ratios, cross-sections, spectroscopic factors, etc.) for nuclei with atomic numbers Z ranging from 1 (H) to 29 (Cu) published in 2005-2010, thus supplementing previous compilations. Additionally, sections of the Introduction are devoted to isomers and nuclear collective excitations, recent trends in the theory of nuclear structure and nucleon interactions (tensor force effects, nonstatistical effects), and γ-ray cascade measurements after neutron capture. In view of the large amount of data available some of the information is given online only at www.springermaterials.com.