1. Introduction

Many variables of interest in forest inventory and monitoring applications cannot be measured directly. Examples include such basic quantities as stem volume where diameters and heights are measured on individual trees and a table or an equation (collectively referred to as a model) is used to estimate the associated volume. The types of models available for estimating quantities like wood volume, biomass, and carbon content, including examples, are given in the following sections. Issues to consider in the choice of model for a specific application, along with modeling objectives and contexts, are also discussed.