Treatment failure in community-acquired pneumonia (CAP) is the failure to normalize the clinical features (eg, fever, cough, sputum production), or nonresolving image in chest radiograph, despite antimicrobial therapy. The incidence of treatment failure in CAP has not been clearly established; according to several studies it ranges between 6% and 15%. The rate of mortality increases significantly, especially in those patients with severe CAP. It is important to be able to identify what patients are at risk for progressive or treatment failure pneumonia that may make them candidates for a more careful monitoring. This doctoral thesis has been structured following the guidelines of the rules for submission of doctoral theses as a compendium of publications, adopted by the Council, Department of Medicine, University of Barcelona. The studies are part of this thesis belong to the same line of research, validated the current guidelines of pneumonia-community acquired and hospital acquired. The results of the studies have provided relevant and innovative in this field and were collected in 2 original articles published in international journals with a widespread global impact factor of 16,37 points.