Abstract        The impact of schema theory on reading comprehension process has been tremendous. Schema theory describes the process by which readers combine their own background knowledge with the information in a text to comprehend that text. All readers carry different schemata (background information) and these are also often culture-specific. This is an important concept in ESL teaching, and prereading tasks are often designed to build or activate the learners' schemata. This paper summarizes some of the research into schema theory and its applications to ESL reading comprehension. Key words: schema theory, background knowledge, reading comprehension, EFL.