The Pneumocystis organism was initially described in 1909 by Chagas. Pneumocystis jiroveci is an uncommon opportunistic organism, which induces a severe and mostly fatal pneumonia in immunocompromised people. Pneumocystosis comes from reactivation of latent infection acquired while childhood or active acquisition while immunosuppression state. The epidemiology of Pneumocystis pneumonia (PCP) has changed considerably through the course of the HIV/AIDS epidemic. The establishments of PCP prophylaxis in 1989 and effective combination antiretroviral therapy in 1996, have resulted in substantial reduces in the incidence of PCP. Due to the insufficient specificity of clinical symptoms as well as the possibility of toxicity with therapy, the affirmation of PCP is important details for correct patient management. The traditional method for diagnosis of PCP depends on the microscopic visualization of organisms in respiratory samples. PCR diagnosis of P. jiroveci was introduced experimentally in 1990. Molecular methods, could detect P jirovecii DNA in respiratory samples from patients without clinically apparent PCP. In this review both epidemiology and diagnosis of Pneumocystis jiroveci will be discussed.