In this PhD thesis, I investigate the impact of grammatical differences on English-Mandarin Chinese simultaneous interpreting (SI), drawing upon an empirical study of professional and student interpreters. The thesis focuses on the effects of three English grammatical categories including passives, adverbials and noun phrases and of three Mandarin Chinese grammatical categories including coverb phrases, noun phrases and topic-prominent clauses on SI between the two languages. For each category, I compare interpretations of instances in which the grammatical structures are the same across the two languages with interpretations of instances in which the grammatical structures differ across the two languages, focusing on accuracy of content and appropriateness of delivery. The results indicate that grammatical differences have a statistically significant impact on the interpreting performance of both professionals and students, although the impact of expertise is also attested through the consistently better performance of professionals than of students.