This briefing note uses an accounting framework, the life-cycle costs approach (LCCA), to explain the main cost components for water and sanitation in rural and peri-urban areas. The briefing note explains the building blocks used in the LCCA, then shows how this approach estimates the true cost of extending sustainable and good quality water and sanitation services to the poorest. The LCCA approach provides the most useful answer to: "What is the cost per year per person of delivering clean water and good sanitation services?" Detailed cost breakdowns are available in the annex.