Bioaerosols commonly occur as airborne particles that consist of, or originate from, living organisms. They are ubiquitous in the environment and have significant health impacts that range from the mild, such as allergic reactions, to the severe, including asthma and cancer. Bioaerosols have unique and varied physical, chemical and biological properties that affect their movement and interactions within their surrounding ecological systems and are generally categorized based on their physical (e.g. micron, submicron, nanometer size) and biological (e.g. viable, nonviable, culturable) properties. These properties directly impact their behavior in air, including how long they remain airborne, their deposition in the respiratory tract, binding with other particles and surfaces, etc. This chapter discusses how to characterize bioaerosols in terms of their particle size distribution, shape, density, surface area, mass and concentration. Equations are presented for calculating particle movement in air that take into account particle relaxation time and terminal settling velocity, a slip correction factor for small particles, dynamic shape factor corrections for irregularly shaped particles and aerodynamic particle size. This information is provided to give the reader the necessary tools to predict the behavior of bioaerosols and to understand better their impact on human health.