This chapter deals with the stereological quantification of structural characteristics of the lung. The aim of design-based stereological methods is the unbiased and efficient estimation of structural features without making any assumptions on the underlying nature of the biological sample. The methods are based on rigorous sampling of location and orientation, the application of appropriate test systems, and the controlling of the precision of the estimates. Here, we describe the workflow from the fixation of the lung over the processing of the tissue samples to gaining estimates on the structural properties of the lung. Specifically, this chapter deals with methods for estimating the reference volume, sampling location, and sampling orientation, estimating volumes and surface areas of alveolar compartments, estimating total alveolar number, performing stereology at light and electron microscopic level, and dealing with technical problems such as tissue shrinkage. The procedures are illustrated using a worked example from the authors' own laboratory.