This is a longitudinal cohort study to assess the impact of repeated sampling of an oral microbiome signature for Barrett's esophagus (BE). Potential participants will be identified through chart review of patients who have had an endoscopy in the past three years.

Saliva collection is non-invasive and can be performed in clinical and non-clinical settings. As such, saliva testing is a highly attractive method for diagnosing and/or monitoring disease. The investigators previously showed that the bacterial make-up ("microbiome") of saliva is highly distinct in patients with and without Barrett's esophagus (BE), a precursor to esophageal cancer that normally can only be diagnosed by undergoing an upper endoscopy. While the microbiome of saliva is felt to be relatively stable over time, it is unclear whether a microbiome signature for Barrett's esophagus is reproducible within individuals. Also, repeated testing of saliva may improve the ability to diagnose Barrett's esophagus.

Inclusion Criteria:

• Scheduled for an upper endoscopy or had upper endoscopy within past three years
• Eighteen years of age or older
• Capable of producing a saliva sample
• Able to give informed consent
• For BE patients only: Endoscopic evidence of Barrett's esophagus (at least 1 cm maximal BE length; i.e. Prague classification: any C, M≥1), and intestinal metaplasia present on esophageal biopsies

Exclusion Criteria:

• History of head and neck cancer or esophageal squamous cell or gastric cancer
• History of esophageal or gastric surgery
• Scheduled to undergo colonoscopy on the day of initial saliva collection
• Scheduled only for Endoscopic retrograde cholangiopancreatography (ERCP) or Endoscopy ultrasound (EUS) without accompanying standard upper endoscopy on the day of initial saliva collection
• For BE patients only: History of prior endoscopic therapy for BE except a history of prior Endoscopic mucosal resection (EMR) of focal lesions without subsequent ablative therapy is permitted

Enrollment will continue until a total of 250 subjects complete all three saliva specimen collections:

• 50 BE patients
• 200 controls

The investigators anticipate that approximately 10% of subjects will not complete all three specimen collections and will be excluded from the primary outcome analyses. Thus, the investigators expect to enroll ~275 subjects to have 250 with three saliva specimen collections. There is no cap on enrollment per study site. The anticipated duration of enrollment is 2 years (24 months).