1. A computer-implemented method of identifying treelet translation pairs for use in a machine translation system that translates a source language input into a target language output, the method comprising: accessing a corpus of pairs of aligned, parallel syntactic dependency structures, each pair including a source language dependency structure having nodes that represent lexical items, the nodes being aligned with nodes representing lexical items in a target language dependency structure; enumerating individual source nodes and combinations of source nodes connected in the source language dependency structure as possible source treelets identifying lexical items, and corresponding dependencies, in the target language dependency structure, that are aligned with the enumerated nodes and combinations of connected nodes, as possible target treelets corresponding to the possible source treelets; extracting well formed treelet translation pairs from the possible source treelets and possible target treelets using a processor of the computer, by extracting as a well formed treelet translation pair the possible source treelet and corresponding possible target treelet only if the lexical items in the possible source treelet are only aligned with lexical items in the possible target treelet and the lexical items in the possible target treelet are only aligned with lexical items in the possible source; storing the treelet translation pairs in a data store; and outputting the treelet translation pairs for use by a component in a machine translation system.