This book demonstrates in detail all phases of the 9th National Forest Inventory of Finland (1996–2003): the planning of the sampling design, measurements, estimation methods and results. The inventory knowledge accumulated during almost one hundred years is consolidated in the book. The purpose of the numerous examples of results is to demonstrate the diversity of the estimates and content of a national forest inventory. The most recent results include the assessment of the indicators describing the biodiversity of forests. The Finnish NFI has been and is a model for many countries worldwide. The methods and results of the book are set in the international context and are applicable globally. The book provides a valuable information source for countries, institutions and researchers planning own inventories as well as modifying the existing ones, or seeking the applicable definitions and estimation methods to use in their own inventories.