The present work has been performed in the Mitochondrial Functionalism Laboratory, Muscle Research Unit, Internal Medicine Department of the Hospital Clínic of Barcelona, located in the Institut d’Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS), in the Faculty of Medicine of the University of Barcelona (UB)- Hospital Clínic of Barcelona, Spain. These studies were initiated in 2007 through a grant from the "Fundación para la Investigación y Prevención del SIDA en España" (FIPSE), being co-tutorized by Dr. Òscar Miró i Andreu, from the Emergency Department of the Hospital Clínic of Barcelona and Dra. Clàudia Fortuny Guasch, associated professor of UB, from the Infectious Diseases Unit, Pediatrics Department of the Hospital Sant Joan de Déu of Barcelona. The obtention of one more grant from FIPSE, within the "Centro de Investigación Biomédica en Red de Enfermedades Raras" (CIBERER), whose Principal Investigator is Dra. Glòria 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.