Biological aerosols can be comprised of bacteria, fungal spores, hyphae pollen, algae, proteins, viruses and fragments of the above. They can have wide-ranging impacts from human disease and allergies to potential impacts on the water cycle by acting as cloud condensation or ice nuclei. Characterization of these populations is desirable to understand which species are important to the variety of impacts. Efforts to characterize these populations of biological particles have used methods such as culture, characterization of nucleic acids and proteins and real-time methods using spectroscopy and mass spectrometry. All characterization methods have limitations and complications that must be considered. Biological particles in the atmosphere can be changed by atmospheric chemical processes in ways that affect their measurement by all the above methods and can affect their viability. The roles of biological aerosols, particularly bacteria, viruses and fungi, in the atmosphere and how they are measured are discussed.