For a long time, the main concern of the pattern recognition research community has been to achieve high accuracy. For example, in the area of Multi-Classifier Systems (MCS), researchers have developed very powerful techniques that combine large number of classifiers through complex combination schemes to achieve satisfactory accuracies. However, this was at the expense of complexity. Even powerful single classifiers (e.g. SVM) have very high complexity. This might give the impression that high accuracies could not be achieved without sacrificing recognition time. Classification cascades are relatively neglected because speed is usually considered a secondary issue by researchers in pattern recognition field. This fact is going to change in the near future because as the world relies more and more on the Internet, web applications are going to include very complex pattern recognition and data mining tasks that are required to be done online. Classification cascades are usually created manually using domain knowledge and are composed in most cases of two or three stages. In this chapter, a model-based algorithm of automatic generation of optimum classification cascades is devised. Given a large pool of classifiers (of size N), it builds a cascade that achieves the lowest possible recognition time while preserving the accuracy of the most powerful classifier in the pool. The proposed algorithm has a low complexity of O(N 2 ) where N is the number of classifiers in the pool. This gives us the freedom of using a large pool of classifiers which leads to more efficient cascades. Other cascade design techniques devised in the literature have very high complexity which hinders using large pool of classifiers.In this chapter we also analyze the performance of the devised algorithm showing its powerfulness and limitations. Also we present an algorithm for building a classification cascade of a given fixed length. This helps building cascades with space complexity constraints and helps in analyzing the performance of the devised algorithm for building optimum classification cascades.