In this paper, an overview about philosophy of science, the concept of "theory" and some characteristics of a "good" theory (1- ordination and explanation of the facts, 2- proposal of problems, and 3- simplicity and funcionality) are initialy introduced, Following, some historical landmarks of acidity and basicity and a summary of the main acid-base theories of the XX century (Arrhenius, solvent systems, protonic, electronic, Lux, Usanovich and ionotropic) are presented. Historical and conceptual relations between these theories are discussed and the three characteristics of a "good" theory are applied. The results showed that the protonic and eletronic theories are the "best", Some discussion of the implications to chemical education are presented too.231126133