This chapter is concerned with the class imbalance problem, which has been recognized as a crucial problem in machine learning and data mining. The problem occurs when there are significantly fewer training instances of one class compared to another class. Most machine learning algorithms work well with balanced data sets since they aim to optimize the overall classification accuracy or a related measure. For imbalanced data sets, the decision boundary established by standard machine learning algorithms tends to be biased towards the majority class; therefore, the minority class instances are more likely to be misclassified. There are many problems that arise from learning with imbalanced data sets. The first problem concerns measures of performance. Evaluation metrics are known to playa vital role in machine learning. They are used to guide the learning algorithm towards the desired solution. Therefore, if the evaluation metric does not take the minority class into consideration, the learning algorithm will not be able to cope with class imbalance very well. With standard evaluation metrics, such as the overall classification accuracy, the minority class has less impact compared to the majority class. The second problem is related to lack of data. In an imbalanced h-aining set, a class may have very few samples. As a result, it is difficult to construct accurate decision boundaries between classes. For a class consisting of multiple clusters, some clusters may contain a small number of samples compared to other clusters; therefore, the lack of data can occur within the class itself. The third problem in learning from imbalanced data is noise. Noisy data have a serious impact on minority classes than on majority classes. Furthermore, standard machine learning algorithms tend to h-eat samples from a minority class as noise_ In this chapter, we review the existing approaches for solving the class imbalance problem, and discuss the various metrics used to evaluate the performance of classifiers. Furthermore, we introduce a new approach to dealing with the class imbalance problem by combining both unsupervised and supervised learning. The rest of the chapter is organized as follows. Section 2 describes the problems caused by class imbalance. Section 3 reviews current stateof- the-art techniques for tackling these problems. Section 4 describes existing classification performance measures for imbalanced data. Section 5 describes our proposed learning approach to handle the class imbalance problem. Section 6 presents experimental results, and Section 7 gives concluding remarks.