This book explores relative clauses (RCs) in Persian and provides an account for these constructions in Head-driven Phrase Structure Grammar (HPSG). Persian RCs are unbounded dependency constructions, all containing the invariant complementizer 'ke'. They also have gaps or resumptive pronouns (RPs), licensed by a higher structure. In some positions only gaps are allowed and in some position only RPs. There are also some positions where both gaps and RPs are allowed. The proposed analysis shows that despite distributional differences between gaps and RPs in Persian RCs, the two show striking similarities. Examples from coordinate structures, parasitic gaps, island constraints, and crossover phenomena provide support in respect of this similarity. Some existing transformational analyses of Persian RCs are reviewed and their shortcomings highlighted. Then, in the constraint-based framework of HPSG, an analysis is proposed that can handle the dependency and the pattern of distribution of gaps and RPs in Persian RCs with a single mechanism which is easily extendable to other types of Persian UDCs, e.g., wh-interrogatives and free relatives.