Ultrafine aerosols (< 100 nm diameter) constitute the largest portion of ambient aerosol loading by number, are highly time and location dependent, and impact both human health and global climate. In order to better understand the sources, transformations, and impacts of aerosol, particle-by-particle chemical composition measurements are required. This review surveys the methods that currently exist for single particle chemical analysis of ambient ultrafine aerosol and discusses the findings of atmospheric measurements using these methods. (C) 2012 Elsevier Ltd. All rights reserved.