Since many decades Uranyl Acetate is used as a main compound for post-staining of ultrathin sections of biological samples to generate contrast for electron microscopy. Usually, this post-staining step is followed by staining with Lead Citrate for contrast enhancement.
Uranyl Acetate and Uranyl Formate are further used for negative staining. Here, the staining agent creates an electron dense coating surrounding the sample enabling TEM imaging. Negative staining is mainly used for TEM visualization of small particles such as macromolecules, extracted cellular organelles, viral particles or other small biological structures.

The radioactivity of Uranyl compounds and the associated legal restrictions were the driving force for the development of new non-radioactive substitutes. Depending on the sample and structures to examine the substitutes show similar staining results.