Community-acquired pneumonia (CAP) is one of the most common causes of hospital admissions and death worldwide. The incidence and mortality of CAP are associated with the presence of comorbidities and increasing age. Streptococcus pneumoniae is the most frequent causative microorganism of CAP, although in many patients with CAP, the causative microorganism remains unknown. Currently, antimicrobial resistance is increasing, so the accurate diagnosis and determination of the causative microorganism are even more important. This is a key point in reducing both morbidity and mortality from CAP, and appropriate antimicrobial stewardship is now a global priority. This review summarizes on the epidemiology, microbiological etiology, and diagnosis of CAP in adults.