Purpose: Todetermine the effectiveness, risk factors for surgical failure, and adverse events in a large cohort of patients receiving stand-alone ab externo poly(styrene-block-isobutylene-block-styrene) (SIBS) microshunt implantation with mitomycin C (MMC) over 3 years of follow-up. Design: Retrospective, interventional case series. Participants: Glaucomatous eyes on maximally tolerated medical therapy with no previous subconjunctival glaucoma surgery. Methods: Records of eyes that underwent ab externo SIBS microshunt with MMC between July 2015 and November 2017 were reviewed. Data from all follow-up visits were utilized and included intraocular pressure (IOP), medication use, postoperative interventions, complications, and reoperations. Main Outcome Measures: The primary outcome was proportion of eyes at 3 years with (1) no 2 consecutive IOPs > 17 mmHg (or < 6 mmHg with > 2 lines of vision loss from baseline); (2) 20% reduction from baseline IOP; and (3) using no glaucoma medications (complete success). Secondary outcomes included 14 and 21 mmHg upper IOP thresholds with and without 20% IOP reduction from baseline, qualified success (with glaucoma medications), risk factors for failure, median IOP/medications, postoperative interventions, complications, and reoperations.