A crucial part of understanding a sentence is to construct its syntactic structure. Without this, it would be very difficult for language users to determine that sentence with different word orders. The processes involved in constructing syntactic structures during language comprehension are commonly referred to as parsing or syntactic processing. Sentence processing theories can roughly be divided into interactive accounts, in which all relevant information can be used immediately; and modular accounts, in which some information can be used immediately but some cannot. Modular models assume that the mind consists of modules which perform very specific processes. In contrast, interactive accounts assume that the processor immediately draws upon all possible sources of information during sentence processing, including semantics, discourse context, and information about the frequency of syntactic structures. In addition to the above, this article discusses syntax and semantics, lexical frequency, discourse effects, working memory capacity, structural complexity, and adoption of ungrammatical syntactic structures.