Outlier detection is a useful technique in such areas as fraud detection, financial analysis and health monitoring. Many recent approaches detect outliers according to reasonable, pre-defined concepts of an outlier (e.g., distance-based, density-based, etc.). However, the definition of an outlier differs between users or even datasets. This paper presents a solution to this problem by including input from the users. Our OBE (Outlier By Example) system is the first that allows users to provide examples of outliers in low-dimensional datasets. By incorporating a small number of such examples, OBE can successfully develop an algorithm by which to identify further outliers based on their outlierness. Several algorithmic challenges and engineering decisions must be addressed in building such a system. We describe the key design decisions and algorithms in this paper. In order to interact with users having different degrees of domain knowledge, we develop two detection schemes: OBE-Fraction and OBE-RF. Our experiments on both real and synthetic datasets demonstrate that OBE can discover values that a user would consider outliers. © 2010 Springer Science+Business Media, LLC.