Expressive comparison of strings is a prerequisite for analysis of sequential data in many areas of computer science. However, comparing strings and assessing their similarity is not a trivial task and there exists several contrasting approaches for defining similarity measures over sequential data. In this paper, we review three major classes of such similarity measures: edit distances, bag-of-word models, and string kernels. Each of these classes originates from a particular application domain and models similarity of strings differently. We present these classes and underlying comparisons in detail, highlight advantages, and differences as well as provide basic algorithms supporting practical applications. © 2011 John Wiley & Sons, Inc. WIREs Data Mining Knowl Discov 2011 1 296–304 DOI: 10.1002/widm.36