One of the difficulties in teaching Marketing, is how to demonstrate the practical applications of a certain number of principles consecrated in the theoretical models, especially those related to the decisions on marketing mix as well as its implementation and evaluation. Marketing practices are developed in a scenario extremely open and it is difficult to obtain an observation deck from where one could see the battle field, where the competitive actions take place. The system dynamics of Marketing indicates that markets are supported by pillars, according to a structured logic in which each fact, action or decision are related to a configured model that encompasses the specific actions and its effects. This paper develops a Simulator for Training Marketing Decisions - STDM, functioning like a bridge between theory and practice. Thru the STDM some quantitative effects can be observed in the market and organizations like the actions to be taken, with the additional advantage that being an hypothetical situation, it can contain in the examples, detailed information that would be inaccessible in the real world, such as sales volume, the internal costs and the economical results of all the competing companies.