In the field of pattern recognition, fusion of multiple classifiers is currently used for solving difficult recognition tasks and designing high performance systems. This chapter is aimed at providing the reader with a gentle introduction to this fertile area of research. We open the chapter with a discussion about motivations for the use of classifier fusion, and outline basic concepts on multiple classifier systems. Main concepts about methods and algorithms for creating and fusing multiple pattern classifiers are reviewed. The chapter closes with a critical discussion of current achievements and open issues.