Wastes are by-products of modern society, which have been treated and disposed of for generations. Instead of being thrown away, some solid wastes are recycled successfully and cost-effectively. The feasibility of recycling is illustrated by the general cycle of matter discussed below. The separation of wastes produces recyclable solid wastes that can be categorized in different groups, and can be treated appropriately. The remaining residues can be either disposed of in a landfill or burned for heat energy recovery. Thus, recycling saves energy and, in some cases, provides energy resources.