This chapter looks at some problems which face the builder of MT systems. We characterized them as problems of ambiguity (lexical and syntactic) and problems of lexical and structural mismatches. We saw how different types of linguistic and non-linguistic knowledge are necessary to resolve problems of ambiguity, and in the next chapter we examine in more detail how to represent this knowledge. In this chapter we discussed instances of lexical and structural mismatches and the problem of non-compositionality (as exemplified by idioms and collocation s) and looked at some strategies for dealing with them in MT systems.