The study of systems with dimensions of the order of a few to a hundred nanometers has attracted great attention. The main reason is the peculiar physical and chemical behavior of these systems; they are specially important in catalysts since metal nano particles are the most common constituents of the supported catalysts. The understanding of the structure of nano particles is a crucial step for the understanding of the catalytic activity. In the present work we report a number of transmission electron microscopy techniques that can be used in order to characterize the shape and structure of nano particles. In the last decade electron microscopy has had a dramatic development and at the present time we have a number of techniques from medium-resolution such as selected zone dark-field (SZDF) and weak beam dark-field (WBDF) as well as direct atom imaging by high resolution. In the present work we discuss the theoretical principles of these techniques and their main applications to the field of nano particle characterization.