Applied linguistics is defined as the theorizing investigation of real-world problems in which language has a central role. Those who regard themselves as applied linguists proceed from the problem to the analysis and the explanation (and possibly the proposals for change), in other words from practice to theory. Those who regard themselves primarily as linguists often proceed in the reverse direction, from linguistic theory to the understanding - and amelioration - of language problems. This further implies that in their applied linguistics, applied linguists take account of a wider range of interests and disciplines than do linguists who are primarily concerned with the development and validation of linguistic theories. Both approaches to applied linguistics, that of linguistics-applied (from theory to practice) and that of applied-linguistics (from practice to theory) are considered in this handbook but greater emphasis is placed on the latter.