The effective size of bioaerosols is the principal determinant of their behavior during takeoff, transport, and deposition. Collection methods involving impaction, impingement, and filtration allow recovery of increasingly small aerosol units and do so “per unit volume of processed air” with more or less well-defined efficiency. These principles offer direction in choosing sampling devices for specific applications; similarly, a particle's appearance, growth potential, and assayability define the scope of relevant analyses. Where a variety of aerosol types occur, multiple collection and/or analytic approaches might be required. The development of a sampling grid in North America makes possible studies of distribution, transport, and climatic effects in ways never previously possible; however, some directive planning and oversight are essential to realize these goals. Methods sensitive to paucimicronic and submicronic particles should be included increasingly in new and ongoing survey protocols. Similarly, available means now facilitate study of aerosols at specific sites, both indoors and outdoors, with estimates of personal exposure during defined events. Resulting data describing airborne prevalence gain special value in light of competent inspection of implicated venues and consideration of alternative sources, such as incursion of bioaerosols into enclosed spaces. (J Allergy Clin Immunol 2003;112:3-8.)