On-site encapsulation is a technique that has been used for centuries as a method for managing and disposing of both nonhazardous and hazardous waste materials. The proper design of an on-site encapsulation system ensures the protection of groundwater resources and prohibits dermal contact by both humans and animals. When suitable treatment methods for certain wastes have not been developed, on-site encapsulation is an effective and economical way to protect both public health and the environment. This paper describes the design techniques used and the problems encountered in the design of an on-site encapsulation system for closure of hazardous waste sites. It includes a brief overview of regulatory requirements; also covered are design parameters, capping systems, and advantages and disadvantages of capping systems. Site-specific factors, weather, space restrictions, and the use of computer modeling software are also discussed. The objective of this paper is to describe the approach the remedial engineer should take in designing an encapsulation system.