This book reports a research program into one of the most controversial questions in the syntax - processing interface: The behavior of the parser at gap positions. While the work done is largely experimental, the results are analyzed both for their relevance to sentence processing and for their implications for competing syntactic frameworks. In particular the differing predictions of PPT and HPSG for structures with dislocated constituents are tested for their empirical adequacy. The author addresses a broad range of questions about gap processing and uses a broad range of methodologies to cut through the confounds which prevent previous work providing clear answers. Wh-movement, scrambling, raising, and equi structures are all addressed, and all current accounts of the experimental evidence evaluated. The results move the debate forward significantly, and provide clear confirmation of some non-trivial claims of generative grammar.