For several years now, hospitals in every region of the nation have reported shortages of registered nurses (RNs). In most cases, however, these shortages have been concentrated in such areas as the intensive care units (ICUs) and operating rooms (ORs). Peter Buerhaus, Douglas Staiger and David Auerbach suggest that part of the explanation for these shortages is the rapid aging of the RN workforce. The aging of the baby-boom generation, coupled with the declining number of young people who choose nursing as a career, has led to the rapid aging of the RN workforce. Currently, 60 percent of the workforce is over 40 years old. In the next ten years, 40 percent of the workforce will be in their 50s and 60s. The authors' analysis of the data suggests that shortages in ICU nurses are partly attributable to the sharp decline of younger-aged RNs from which ICUs have historically drawn the bulk of their RN staffs. Shortages in ORs reflect the fact that the hospital-based degree programs which traditionally supplied most OR staffs are disappearing, and their graduates are reaching the age where they are beginning to reduce their responsibilities or are retiring. This article is the first in a series of four examining the interplay of several factors contributing to current RN shortages, and it focuses on the effects of changes in the age composition of the RN workforce. Other articles in this series examine the impact of the large number of RN graduates from associate degree programs, the impact of the expansion of career opportunities for women and ideas for strengthening the RN workforce.