This summary paper describes the hydrogeologic setting and site history for the Los Alamos National Laboratory (LANL), and as such, serves as an introduction to this special section in Vadose Zone Journal on the LANL site. Since 1943, LANL has operated on the Pajarito Plateau of north-central New Mexico, performing national security research and development, particularly involving nuclear weapons design and engineering. There is an ongoing environmental characterization and remediation program as a result of the legacy of these operations. The flow and transport processes in the thick vadose zone in this semiarid region have been examined extensively at this site, and significant scientific findings have advanced our understanding and predictive abilities. This special section highlights the noteworthy field and modeling investigations conducted at LANL. The authors believe that these articles will be of benefit to the community of hydrologists working on vadose zone flow and transport by providing complex observational data sets and accompanying modeling studies to interpret the field data.