Understanding a text requires not only understanding the individual words and sentences, but also requires the construction of an integrated model of the text as a whole: a Mental Model (Johnson-Laird, 1983) or Situation Model (Kintsch, 1998). In the first part of this paper, we differentiate between the types of inference that occur as a reader understands a text: necessary inferences (at both the local and global level) and ‘merely elaborative’ inferences, which might embellish the reader’s understanding, but which are not essential to it. We then go on to discuss the problems of children who have a Specific Comprehension Difficulty (i.e. they are able to read words at an age-appropriate level but, nevertheless, have a poor understanding of the text overall). We describe the particular difficulties that such children have in answering inferential questions about a text, and outline the evidence that such difficulties are causally related to comprehension skill. We then discuss the reciprocal relation between vocabulary skills and inference making. Inference skills have a clear role in helping readers to derive the meanings of unknown words from text through the use of contextual cues and, conversely, deep vocabulary knowledge (what is known about words), and rapid access to that knowledge, can support inference making.