The objective of this chapter is to introduce the chest computed tomography for the evaluation of respiratory diseases. In particular, the assessment of structural changes of the respiratory system and the related alterations in function represents a fundamental approach to the clinical work-up and the ensuing clinical decisions. The present chapter will provide an account on some of the most important and updated applications of chest CT. Evolution of CT and applications to the respiratory tract, main indications, applications to lung cancer, applications to diffuse parenchymal lung diseases, pattern of increased pulmonary density, pattern of decreased pulmonary density and virtual bronchoscopy are also covered in this chapter. High-resolution CT is the reference imaging method for the study of diffuse infiltrative lung diseases. It has enormously contributed to the long and cumbersome task of classifying this complex range of disorders, differing for pathogenesis and natural history but sharing the involvement of a target structure, the secondary pulmonary lobule.