This book is an up-to-date version of my doctorate thesis at the University of Pittsburgh, under the supervision of professors Gaea Leinhardt, Stephen Koziol, Isabel Beck, Robert Dekeyser, and Richard Donato. This study was undertaken to investigate the written performance of ten foreign students - five native Arabic speakers and five native Spanish speakers - at the University of Pittsburgh. The purpose was to better understand the role of grammar knowledge in the written productions of foreign students. In addition, selected cognitive and cultural aspects of the subjects' written performance were examined.