Pneumonia acquired in the community (community-acquired pneumonia) has a different pathobiology as opposed to pneumonia acquired in the hospital (nosocomial pneumonia). Initial antimicrobial therapy of community-acquired pneumonia is almost always empirical owing to the fact that the results of microbiological testing are not immediately available. As well, for reasons that will be discussed, microbiological testing is successful in isolating the specific organism in just over a third of all cases. Given these limitations, it is necessary to be aware of the spectrum of pathogens that predominate in patients with certain specific comorbidities, occupations and exposures. This chapter discusses the principles of the diagnosis of community-acquired pneumonia and gives due consideration to the many conditions that frequently mimic it. The chapter also attempts to provide insight into the specific etiological agents of community-acquired pneumonia that are possible in a given situation. A discussion on the varied radiological and CT-manifestations, and their relevance is also included.