© 2015 American Association of Endodontists. Introduction A preliminary study done by a National Dental Practice-Based Research Network precursor observed that 44% of general dentists (GDs) reported always using a rubber dam (RD) during root canal treatment (RCT). This full-scale study quantified the use of all isolation techniques, including RD use. Methods Network practitioners completed a questionnaire about isolation techniques used during RCT. Network enrollment questionnaire data provided practitioner characteristics. Results One thousand four hundred ninety of 1716 eligible GDs participated (87%); 697 (47%) reported always using an RD. This percentage varied by tooth type. These GDs were more likely to always use an RD, do not own a private practice, perform less than 10 RCTs/month, and have postgraduate training. Conclusions Most GDs do not use an RD all the time. Ironically, RDs are used more frequently by GDs who do not perform molar RCT. RD use varies with tooth type and certain dentist, practice, and patient characteristics.