Call centers have emerged as the primary vehicle for firms to interact with consumers, transforming consumer service jobs once characterized by variety and personal relationships into routinized and high speed operations. Call centers are used to provide services in many areas and industries: banks, insurance companies, emergency centers, information centers, help-desks, tele-marketing and more. Technological development has allowed remote service delivery using various channels of telecommunication. The definition of a call center is continuously changing, but the core fundamentals of a customer making a call (via a phone, email, web site, fax or Interactive Voice Response) to a center (collection of resources) will remain constant. Call center, contact center or customer interaction center operate on identical principals of meeting customer needs in real-time or near real-time. The goal of the present thesis is to contribute to the operations management research of call centers. We aim to enhance our understanding of such complex systems, so as we gain useful guidelines for the practitioners.