Dr. Lisa M. Harvey, a member of the Biology Department at Victor Valley College (California), notes that studies with rats, mice, and humans suggest that a distinctive scent for each subject may derive from genes in the Major Histo-compatibility Complex (MHC). Other factors contributing to the distinctiveness of a person's scent include nutrition and hygiene as well as a person's health. A study conducted by G.A.A. Schoon of the Netherlands National Police Agency and Leiden University examined how the age of odor trace evidence affects the performance of Dutch and German scent-identification dogs. Although the dogs performed flawlessly in matching odors collected on the same day, their performance dropped when distinguishing scent evidence stored for 2 weeks. After this initial drop in scent detection, however, aging did not significantly diminish the dogs' performance even up to 6 months. Dogs' ability to detect and distinguish scents can aid police in tracking or trailing an individual or in connecting a suspect with evidence in a scent lineup. Dogs track the direction of a person's movements by following the odor of crushed vegetation and disturbed ground caused by human contact. Tracking dogs can also use traces of fresh generic human scent to help in their pursuit. Trailing a specific person and identifying an individual in a scent lineup require a scent sample of the subject of interest. This article describes how to organize a scent lineup and briefly reviews the legal status of scent evidence in various jurisdictions. 4 references