Modern data on diatom algae - unicellular microorganisms with siliceous frustule composing Class Bacillariophyceae, are reviewed. Brief characteristic of biology, morphology, frustule structure and its elements, principles of diatom systematics, and also biogeography and ecology are given. A role of diatoms in sedimentation and peculiarities of preservation of their remains in bottom sediments are discussed. Special attention is paid to issues of effective application of data on fossil diatoms for detailed biostratigraphic subdivision, dating and correlation of sedimentary successions (first of all, marine deposits) as well as for paleoenvironmental reconstructions. General characteristic of diatom zonations proposed for marine Cretaceous and Cenozoic sediments is given. Main features of diatom evolution are discussed.