Diatoms or Bacillariophyoeae are silicious algae, the test of which is composed of a hypotheca (box) and an epitheca (lid). In this class two orders can be distinguished: the Centrales and the Pennales. The morphology and structure of the test are treated in detail. The silicious algae are very important with respect to the nutricion circle in the water, biological water purification, stratigraphical age and ecological determinations. The stratigraphy and ecology are illustrated by some examples.