Background: Acid base disorders are frequently present in critically ill patients. Metabolic acidosis is associated with increased mortality, but it is unclear whether as a marker of the severity of the disease process or as a direct effector. The understanding of the metabolic component of acid base derangements has evolved over time, and several theories and models for precise quantification and interpretation have been postulated during the last century. Unmeasured anions are the footprints of dissociated fixed acids and may be responsible for a significant component of metabolic acidosis. Their nature, origin, and prognostic value are incompletely understood. This review provides a historical overview of how the understanding of the metabolic component of acid base disorders has evolved over time and describes the theoretical models and their corresponding tools applicable to clinical practice, with an emphasis on the role of ABG in general and several specific settings. Objectives: Aim of the study : to determine the value of ABG on mortality and morbidity in Pediatric Intensive Care Unit in minia university hospital. Methods: This was a retrospective study done over 2 years. There were total 653 patients (1 month to18 years), who were divided into survivals and non survivals groups. All data were collected and analyzed retrospectively including: Name, Age, sex, , length of stay in PICU, underlying etiology, cause of admission, and requirement for mechanical ventilation. laboratory investigations; from the patients' records, we obtained the results of routine blood samples on patients' admission to PICU which were immediately transported to the central laboratory in the hospital. These samples were arterial blood gases, Data retrieved from the medical files and statistically analyzed after permission from the ethical committee. Results: There were significant difference in the outcome of patients admitted to the PICU regarding the age group, sex, length of stay in PICU, requiring mechanical ventilation, respiratory failure P-value were (<0.001, <0.001, 0.007, <0.001, and <0.001 respectively). Conculsion:. There is no consensus regarding preferred methodology for the evaluation of acidebase derangements in critically ill patients. The physicalechemical approach by Stewart does not have a clear advantage over the traditional bicarbonate-based method. Both should be regarded as complementary for optimal understanding of acidebase disorders. Keywords; ABG, metabolic acidosis, outcome in children admitted on PICU.