|Abstract:||This thesis investigates the effect of speech rate on Mandarin tone sandhi. This effect is argued to be the result of hypothesized thresholds on the duration of junctural units that lie between phonological phrases. A new type of constraint, *WrapSD(nmsec), is proposed that assigns a violation to every juncture that exceeds a specified durational threshold. The *WrapSD(nmsec) constraints are incorporated within recent constraint-based approaches to phonological phrasing and shown to be superior to alternative approaches that require either cyclic rule application or constraint re-ranking. This approach also leads to an empirical exploration of four new sentence types, which are documented with eleven native Mandarin speakers and shown to be consistent with juncture thresholds.