When data objects that are the subject of analysis using machine learning techniques are described by a large number of features (i.e. the data is high dimension) it is often beneficial to reduce the dimension of the data. Dimension reduction can be beneficial not only for reasons of computational efficiency but also because it can improve the accuracy of the analysis. The set of techniques that can be employed for dimension reduction can be partitioned in two important ways; they can be separated into techniques that apply to supervised or unsupervised learning and into techniques that either entail feature selection or feature extraction. In this paper an overview of dimension reduction techniques based on this organisation is presented and representative techniques in each category is described.