The term interstitial lung disease (also called diffuse infiltrative lung disease) encompasses a heterogeneous group of processes characterized by the appearance of an inflammatory reaction in the alveolar wall that can be triggered by different antigens. This group of diseases represents a wide spectrum of processes of diverse etiologies, and sometimes the nomenclature can be confusing. High-resolution computed tomography (HRCT) is the imaging method of choice for the evaluation and diagnosis of interstitial lung diseases because it confirms the presence of lung disease and establishes the correct diagnosis for associated complications. Nevertheless, the definitive diagnosis of these entities requires the imaging findings to be interpreted together with their clinical manifestations and histological confirmation. In this group of diseases, HRCT findings play a fundamental role, being especially important for avoiding unnecessary biopsies. For these reasons, clinicians need to be familiar with the basic radiologic patterns associated with this group of lung diseases: septal, reticular, nodular, ground-glass, cystic, and consolidations. This chapter describes the features of these patterns and ways that they can present, and it reviews some of the most common interstitial lung diseases, emphasizing the predominant radiologic patterns in each of them.