IMPORTANCE: Glaucoma is a significant health problem for which diagnosis remains suboptimal. Optic disc evaluation, which is fundamental to the diagnosis, is a difficult skill to acquire. OBJECTIVES: To determine the optic disc characteristics that most influence decision making in the assessment of glaucoma likelihood and to ascertain the optic disc features associated with overestimation and underestimation of glaucoma likelihood. DESIGN, SETTING, AND PARTICIPANTS: This prospective, observational, Internet-based study with multinational participation included 197 ophthalmic clinicians (37 glaucoma subspecialists, 51 comprehensive ophthalmologists, and 109 ophthalmology trainees) from 22 countries who self-registered for the Glaucomatous Optic Neuropathy Evaluation (GONE) Project from December 1, 2008 through June 30, 2010. INTERVENTIONS: A series of 42 monoscopic optic disc photographs of healthy and glaucomatous eyes were presented to clinicians using the GONE Project Program. Participants were asked to assess each disc according to 9 conventional topographic features and assign a presumptive grade for glaucoma likelihood. MAIN OUTCOMES AND MEASURES: Agreement (κ and weighted κ) among participants for disc signs and glaucoma likelihood and contributions of disc-related factors to overestimation and underestimation of glaucoma likelihood. RESULTS: Ophthalmology trainees and comprehensive ophthalmologists underestimated glaucoma likelihood in a mean (SD) of 22.1% (1.6%) and 23.8% (1.8%) of discs, respectively. Underestimation of vertical cup-disc ratio and failure to identify retinal nerve fiber layer loss, disc hemorrhage, or rim loss were most likely to lead to underestimation of glaucoma. When all 4 features were inaccurately assessed, underestimation of glaucoma likelihood increased to 43.0%. Ophthalmology trainees and comprehensive ophthalmologists overestimated glaucoma likelihood in a mean (SD) of 13.0% (1.2%) and 8.9% (1.3%) of discs, respectively. Overestimation of glaucoma likelihood was associated with overestimation of retinal nerve fiber layer loss, rim loss, vertical cup-disc ratio, disc hemorrhage, and incorrect assessment of disc tilt and was more likely in large discs. CONCLUSIONS AND RELEVANCE: Ophthalmology trainees and comprehensive ophthalmologists underestimated glaucoma likelihood in approximately 1 in 5 disc photographs and were twice as likely to underestimate as overestimate glaucoma likelihood. Underestimating the vertical cup-disc ratio and cup shape and missing retinal nerve fiber layer defects and disc hemorrhage were the key errors that led to underestimation. When all 4 parameters were incorrectly assessed, underestimation increased to almost 1 in 2.