In this paper we tried to summarize the main features of the recent theoretical developments to study the behavior of the nominal exchange rate under target zones. We first presented Krugman's canonical model and exposed the properties of the exchange rate system ( exchange rate stability with respect to flexible regime and some freedom to the monetary policy respect to fixed regime). Due to the importance of "credibility" in the development of the above mentioned properties, we tried to deepen into its determinant variables. Thus, using Krugman and Rotemberg's developments about the collapse of the system and those of Bertola and Caballero about the "realignments" of the zone, we tried to enhance the importance of an accurate amount of reserves of the Monetary Authority to achieve an acceptable level of "credibility" needed to maintain the properties of the system.