Objectives: Questionable occlusal caries (QOC) can be defined as an occlusal tooth surface with no cavitation and no radiographic radiolucencies, but caries is suspected due to roughness, surface opacities, or staining. Previous results from this study suggest that 1/3 of patients have a QOC. The objective of this report is to quantify the characteristics of these common lesions. Methods: A total of 82 dentist and hygienist practitioner-investigators from the United States and Denmark in The National Dental Practice-Based Research Network participated. When consented patients, age 6 or older, presented with a QOC on a permanent tooth, information was recorded about the patient, tooth, lesion, and treatments. Results: 2,669 QOC from 1,753 patients were recorded. Lesions were usually associated with a fissure, on molars, and yellow to black in color. Half presented with a chalky luster and had a rough surface when examined with an explorer. There was an association between color and luster; a higher proportion of chalky than shiny lesions were light (22% vs. 9%; p<0.001). Lesions light in color were less common in adults than pediatric patients (9% vs. 32%; p<0.001). Considering color-luster combinations; chalky-light (active lesions), shiny-dark (inactive lesions), and mixture (activity - intermediate), 10% were active, 47% inactive, and 42% intermediate. Active lesions were more common among pediatric than adult patients (22% vs. 6%, p<0.001). Conclusions: This is the first study to investigate characteristics of QOC in routine clinical practice. Clinicians commonly face this diagnostic uncertainty. Determining the characteristics of these lesions is relevant when making diagnostic and treatment decisions. QOC lesion color, luster, and activity significantly differed between adult patients and pediatric patients.