Expedited partner therapy (EPT) is a strategy for treating the partners of chlamydia index cases by which a health care provider gives an index patient drugs or a prescription for treatment of chlamydia to deliver to their sex partner without an intervening medical evaluation of the partner. Despite routine offer of EPT in New York City Department of Health and Mental Hygiene (DOHMH) sexual health clinics, the majority of patients who are eligible for EPT do not receive it, largely because EPT eligibility requires lab confirmation of chlamydial infection, which is lacking in situations where patients are treated for chlamydia on the same day they are tested for chlamydia (i.e., presumptive treatment). These patients become eligible for EPT after they leave the clinic and often do not return for EPT. This dissertation includes three papers: one systematic review and two original analyses. The objective of the systematic review was to synthesize existing estimates of EPT effectiveness to better understand the impact of biases on these estimates; a meta-analysis provided an aggregate estimate of the effectiveness of EPT for preventing index patient reinfection with chlamydia and/or gonorrhea. We found 6 studies that included some measure of EPT effectiveness. Meta-analysis revealed that EPT significantly reduced the risk of reinfection from chlamydia and/or gonorrhea, but it also revealed a substantial amount of heterogeneity. Systematic review revealed that inclusion of patients whose sex partners were at the clinic or already treated for infection was a common source of bias among existing estimates of EPT effectiveness. The two original analyses used data from NYC DOHMH sexual health clinics where EPT is routinely offered as patient delivered partner therapy (PDPT), a form of EPT where medication is given directly to the index patient. The objective of the first analysis was to identify predictors of presumptive treatment and predictors of being offered PDPT among patients eligible for PDPT in the NYC clinics. This analysis demonstrated that patient diagnosis as a contact to a sexually transmitted infection (STI) that would warrant treatment with azithromycin or doxycycline (termed STI contacts) was the best predictor of presumptive treatment in NYC DOHMH sexual health clinics. Patients who were not contacts to such STIs or who were STI contacts with more than one sex partner were more likely to be offered PDPT compared to patients who were STI contacts and reported ≤ 1 sex partner. Males not diagnosed as STI contacts were identified as a target population for increasing rates of PDPT offer. The objective of the last analysis was to provide an estimate of PDPT effectiveness for preventing index patient reinfection with chlamydia. This analysis was novel compared to existing estimates of EPT effectiveness in that we excluded patients whose sex partners were at the clinics at the time of chlamydia testing, treatment, or PDPT offer. We found that PDPT significantly reduced the risk of repeat chlamydial infection at both 6 months and 1 year after initial infection. This result was unchanged by multiple sensitivity analyses that assessed the validity of our estimate. In this dissertation, we were able to fill gaps in the literature regarding EPT implementation. The results may help to decrease missed opportunities for offering patients EPT and to support the continued scale-up and optimization of EPT.