The present work has been performed in the Mitochondrial Functionalism Laboratory, Muscle Research Unit, Internal Medicine Department of the Hospital Clinic of Barcelona, located in the Institut d'Investigacions Biomediques August Pi i Sunyer (IDIBAPS), in the Faculty of Medicine of the University of Barcelona (UB)- Hospital Clinic of Barcelona, Spain. These studies were initiated in 2007 through a grant from the "Fundacion para la Investigacion y Prevencion del SIDA en Espana" (FIPSE), being co-tutorized by Dr. Oscar Miro i Andreu, from the Emergency Department of the Hospital Clinic of Barcelona and Dra. Claudia Fortuny Guasch, associated professor of UB, from the Infectious Diseases Unit, Pediatrics Department of the Hospital Sant Joan de Deu of Barcelona. The obtention of one more grant from FIPSE, within the "Centro de Investigacion Biomedica en Red de Enfermedades Raras" (CIBERER), whose Principal Investigator is Dra. Gloria Garrabou Tornos, has permitted the continuation of the studies in the last period of this work. This book entitled: "Mitochondrial function in HIV-infected children" has explored the mitochondrial status in this population group.