In call centers it is crucial to staff the right number of agents so that the targeted service levels are met. These staffing problems typically lead to constraint satisfaction problems that are hard to solve. During the last decade, a beautiful manyserver asymptotic theory has been developed to solve such problems for large call centers, and optimal staffing rules are known to obey the square-root staffing principle. This paper presents refinements to many-server asymptotics and this staffing principle for a Markovian queueing model with impatient customers. © 2012 INFORMS.