We are very pleased to guest edit this special issue of Queueing Systems, in honor of Ward Whitt upon reaching 75. This special issue is a companion to the 21st Applied Probability Day hosted by the Center for Applied Probability at Columbia University in 2017. In fact, there were 2 days: April 21 with senior guest speakers and April 22 with Ward’s recent students. The speakers were invited to submit papers to this special issue.

The speakers on April 21 were: Soren Asmussen, Peter Glynn, Avi Mandelbaum, William Massey, and Marty Reiman. The speakers on April 22 were: Rouba Ibrahim, Guodong Pang, Yunan Liu, Itai Gurvich, and Song-Hee Kim. The range, depth, and breadth of topics evidences the large footprint Ward has left on the queueing field. That footprint begins with his seminal Ph.D. work on weak convergence for queues in heavy traffic under the guidance of Donald L. Iglehart, and continues to his later work with Joe Abate and other colleagues on numerical transform inversion, and his book, Stochastic Process Limits, that many consider a “bible” for the heavy-traffic community. However, this skips hidden unpublished gems like Ward’s homework solutions for the Erlang-B and Erlang-C models from 2002, which only can be found on his web page.

Ward’s influence on the queueing community and beyond is large, both through his research and through his mentoring. Indeed, we have both benefitted from his mentoring. We are honored to bring you this special issue that highlights some current research influenced by Ward’s work. We begin this issue with a short paper by Ward putting the work in this issue into a broader context.