Transmission electron microscopy (TEM) allows ultrastructural imaging of biological specimens. The precondition is a sufficient thinness of samples, such as macromolecules, cells, or tissue sections with a thickness of less than 100 nm (see also "Cutting"). Depending on the question, a TEM sample could be patient material (biopsy) for the diagnosis of diseases or a research issue, such as the investigation of cellular, viral or bacterial ultrastructures. For the scientific analysis, for example, macromolecules like DNA can be selectively purified and concentrated by ultracentrifugation and gradient fractionation (BioComp Fractionator). Afterwards, further steps are usually necessary, such as fixation, dehydration and embedding, and in the case of cells or tissues, preparation of sections by ultramicrotomy, followed by contrasting of the ultrathin slices.