Homology-directed DNA repair (HDR) necessitates templated DNA synthesis to repair double-strand breaks (DSBs), replicative lesions, and telomeres. Alternative lengthening of telomeres (ALT) is a clinically relevant model of HDR responsible for telomere maintenance in ~10-15% of human cancers. However, the mechanisms of mammalian HDR synthesis at telomeres or elsewhere in the genome were largely unknown. Here, we discovered that telomere DSBs elicit robust templated synthesis capable of generating long nascent telomeres, a process we named break-induced telomere synthesis (BITS). BITS and spontaneous telomere repair synthesis proceed through an alternative form of HDR independent of RAD51 that requires DNA polymerase delta, including its POLD3 subunit. RAD52 controls spontaneous telomere repair synthesis but is dispensable for BITS, suggesting lesion specific pathway requirements. Additionally, telomere maintenance is disrupted in ALT cells deficient in POLD3 or RAD52, demonstrating that telomere repair synthesis underlies ALT. While POLD3 emerged as a central mediator of numerous DNA repair synthesis processes, its protein interaction network was largely unknown. To address this, we purified the POLD3 replication complex and uncovered novel interactions predicted to impact replication and repair. Taken together, this thesis provides some of the first mechanistic insights into mammalian HDR synthesis as well as a framework for continued studies of DNA repair synthesis and telomere maintenance.