The information on Mira-type stars and stars adjacent to them at the Hertzsprung — Russel diagram is presented. A detailed description of their observational characteristics is given. We give a survey of important observational works concerning: multicolor photometry with special attention to the IR emission, maser emission, shock waves, mass loss, binarity, the problem of the pulsational mode, direct measurements of angular and linear dimensions, statistic investigations, study of kinematic characteristics etc. The most interesting problems regarding long periodic variable stars are specified. Some attention is given to the classification and evolutionary stage of these objects.