This dissertation investigates preposition stranding observed in Colloquial Welshusing agenerative grammarapproach.Welsh traditionally disallows P-stranding. In Literary Welsh, prepositional relatives require a resumptive pronounwhich is licensed by the rich agreement of P, and wh-questions requirepied-piping of the entire PP.However, the use of uninflected stranded preposition can be found colloquially, both in relatives and wh-questions.This dissertation proposes an account ofthe different syntactic behaviour between the two varieties, based on the notion of PF feature checking proposed by Ackema and Neeleman (2004). I claim that if PF checking takes place between aP head and its DP complement, the DP will be unable to move out of the complement position. This means that P-stranding is impossible. On the other hand, if PF checking does not hold between P and its complement,P-stranding is possible.Chapter 1 provides theoretical assumptionsand background of previous research on P-stranding.Chapter 2 describes wh-constructions in Welsh and investigates the (un)availabilityof P-stranding inthe literary and colloquial varieties. Chapter 3 considers formal properties of Welsh resumptives. Following Willis (2011), I assume that Welsh wh-dependencies with resumptive pronouns obey successive cyclicity. Chapter 4presents the PF feature checking approach toP-stranding. I assume that the crucial difference between the two varieties is that P in Literary Welsh possesses AGR-features, but in Colloquial Welsh does not. I claim that the availability of PF checking regulates the availability of P-stranding.Chapter 5investigates P-stranding generalizations, observed in Abels (2003),on pseudo-passives, clitics, verbal particlesand sluicing. Chapter 6 discusses diachronic implications of the occurrence of P-stranding. I considerhow the P-stranding option cameinto Welsh grammar, using the notion of bilingual mode developedby Grosjean (2001).