Alignments constitute one of the processes commonly used to compare strings. They allow to visualize the resemblance between strings. This chapter deals with several methods that perform the comparison of two strings in this sense. The extension to comparison methods of more than two strings is delicate, leads to algorithms whose execution time is at least exponential, and is not treated here.

Alignments are based on notions of distance or of similarity between strings. The computations are usually realized by dynamic programming. A typical example used for the design of efficient methods is the computation of the longest subsequence common to two strings. It shows the algorithmic techniques that are to implement in order to obtain an efficient computation and to extend possibly to general alignments. In particular, the reduction of the memory space obtained by one of the algorithms is a strategy that can often be applied in the solutions to close problems.

After the presentation of some distances defined on strings, notions of alignment and of edit graph, Section 7.2 describes the basic techniques for the computation of the edit (or alignment) distance and the production of the associated alignments. The chosen method highlights a global resemblance between two strings using assumptions that simplify the computation.