Chest X-Ray is the most accurate method of imaging for infectious diseases in an immunocompetent patient. Computed tomography (CT) may be useful in certain circumstances, particularly in case of atypical findings at the time of diagnosis or in case of complications. CT helps to detect and perform a complete study of the lesions, some aspects being very suggestive of a diagnosis, as in post-primary active tuberculosis. CT may also detect an unknown underlying etiology. Multiplanar reformations with helical CT can be useful for example in case of empyema. In case of non tuberculous bacterial infections, CT is mainly recommended when abscess and empyema are difficult to differentiate or in case of pleural complications with possible percutaneous treatment. In case of tuberculosis, CT may be indicated when clinical and chest X-Ray findings are discordant, in case of mediastinal adenopathies, when reactivation is suspected or in case of complications as hemoptysis. A baseline CT examination could be proposed at the end of a specific treatment to facilitate the diagnosis of reactivation tuberculosis. A nontuberculous mycobacterial infection should finally be suspected in front of peculiar CT findings.