Comprehension of idioms is the act of processing and understanding idioms. Idioms are a common type of figure of speech. Based on common linguistic definitions, an idiom is a combination of words that contains a meaning that cannot be understood based on the literal definition of the individual words. An example of an idiom is hit the sack, which means to go to bed. It can be used in a sentence like the following: I'm so exhausted that I'm going to hit the sack now. Comprehension of idioms is the act of processing and understanding idioms. Idioms are a common type of figure of speech. Based on common linguistic definitions, an idiom is a combination of words that contains a meaning that cannot be understood based on the literal definition of the individual words. An example of an idiom is hit the sack, which means to go to bed. It can be used in a sentence like the following: I'm so exhausted that I'm going to hit the sack now. Traditionally, idiom comprehension was thought to require a distinct processing mode other than literal language comprehension. Subsequent research suggested that the comprehension of idioms could be explained in the context of general models of comprehension. Contemporary researchers have also posited that different modes of processing are required for distinct types of idioms. Factors, such as idiom familiarity, transparency, and context are found to influence idiom comprehension. Recent neurolinguistic research has found, using various techniques, several neural substrates that are associated with idiom comprehension, such as the left temporal lobe and prefrontal cortex. Psycholinguistic research in idiom comprehension began in the 1970s. Early research on figurative language typically assumed that comprehending figurative and literal language involved different kinds of processes. The non-compositional models of idiom comprehension were constructed based on this assumption. Non-compositional models in general assume that idioms are stored like long words in memory. For example, the entire phrase kick the bucket is represented by the meaning to die. The words kick and bucket separately do not contribute to the idiomatic meaning. Since individual words do not matter in an idiomatic expression, they are thought to be processed together as one entity during comprehension. In contrast, processing a literal expression requires the meaning of each word to be derived and then perceived in relation to the other words in the phrase. Non-compositional models include the literal first hypothesis, lexical representation hypothesis, and the direct access hypothesis. The Literal First Hypothesis The Lexical Representation Hypothesis The Direct Access Hypothesis There are several criticisms for the non-compositional models. First, studies demonstrated that idiom expressions are not processed more slowly than literal expression. In fact, it is often the opposite, which is in contrary to the prediction of literal first hypothesis. Second, idioms have been found to be more than just “frozen phrases” or long words. For example, some idioms can be transformed to some extent and still be recognized and understood. For example, spill the beans can be used as “the beans were spilt by Mary”. This is possible because spill the bean can be mapped on the meaning “reveal the secret”, i.e. spill (reveal) and beans (secret). Such idiom shows that the internal structure of the word string matters during comprehension. The meaning of some idioms, like play with fire, can be also inferred from literal interpretation of their components (i.e. to do something dangerous). These findings reveal that idioms are not a homogeneous, distinct group and thus may not involve different processing strategies from those for literal expressions.