Esri's MapObjects Internet Map Server (IMS) has quickly become one of the most popular technologies for Internet interactive mapping applications. The United States Environmental Protection Agency (USEPA) has developed and deployed several large-scale MapObjects IMS applications on the Internet with satisfactory scalability (http://www.epa.gov/enviro/enviromapper.html). This article discusses many lessons related to the development and deployment of scalable MapObjects IMS applications on the Internet. Since Microsoft Visual Basic (VB) is the primary programming environment for the Esri MapObjects IMS team and many other MapObjects IMS application developers, this article focuses on the development of scalable MapObjects IMS applications using Microsoft Visual Basic. The implication of MapObjects IMS as a single-threaded VB component is demonstrated. Details on how to utilize a multi-CPU windows NT system with single-threaded MapObjects IMS applications are given, and several technical areas important to the implementation of a scalable MapObjects IMS application are discussed.