This work is an investigation into part of the human sentence parsing mechanism (HSPM), where parsing implies syntactic and non-syntactic analysis. It is hypothesised. that the HSPM consists of at least two processors. We will call the first processor the syntactic processor, and the second will be known as the non-syntactic processor. For normal sentence processing, the two processors are controlled by a 'normal component", whilst when an error occurs, they are controlled by an "error recovery component". These divisions are based on the observation that human beings are able to bring at least two distinct types of information to bear on a text. The resolution of lexical ambiguity will be used as a vehicle to investigate this hypothesis. Under control of the normal component, the syntactic processor is unconscious, deterministic and fast, but limited. It is hypothesised that the syntactic and Non-syntactic processors work in parallel during the processing of a normal sentence. During processing of some sentences, the syntactic processor will, at key points, ask the non-syntactic processor to make a decision in order to resolve an ambiguity. These key points occur whenever a situation arises in which the syntactic processor can no longer guarantee a correct analysis. A major focus of this research is the identification of those situations in which people use the non-syntactic processor to assist with the resolution of ambiguity and the sentences in which this occurs. When both the syntactic and non-syntactic processors fail, for example during the processing of a garden path sentence, it is hypothesised that an ''error recovery component" is used, which controls both processors and is slower, semi-conscious and non-deterministic. We are concerned. with modeling only the normal use of the syntactic processor. The major test of the psychological validity of such a model is that it fail on precisely those sentences that humans find to be garden paths. We use, as a starting point, Marcus's work on deterministic parsing. The advances reported here are: -Reaction time experiments are used to provide a non-subjective classification of sentences as garden paths or not. Using this classification, it is shown that Marcus's parser would succeed on some garden path sentences and fail on some non-garden path sentences. - This deficiency can be corrected. by the use of non-syntactic information for ambiguities which may lead to a garden path. - Non-syntactic information is to be used to help resolve an ambiguity when the syntactic processor can no longer guarantee a correct analysis. All other ambiguities are to be resolved on the basis of syntactic information. - An amended parser, ROBIE is presented which incorporates these conclusions. ROBIE is shown to be compatible with the psychological evidence currently available on human sentence comprehension.