Edentulism in the posterior maxilla can present with compounding variables that make it a difficult region to restore with implants. Pneumatization of the sinus floor is typically accounted for during surgical treatment planning, but other factors such as horizontal ridge deficiency and vertical defects may be overlooked. This report reviews the different classifications used to treat the posterior maxilla and introduces a new system that incorporates all factors critical for implant success. Class A represents abundant bone with >= 10 mm bone height below the sinus floor and >= 5 mm bone width, allowing proper implant placement. Class B indicates barely sufficient bone with 6 to 9 mm bone height below the sinus floor, and this can be further subclassified into division h (horizontal defect; 5 mm bone width), division v (vertical defect; > 3 mm away from cementoenamel junction), and division c (combined horizontal and vertical defect). Class C indicates compromised bone with = 5 mm bone height below the sinus floor, and this can also be subclassified similar to Class B. The ABC classification is a simple system to guide clinicians in proper implant treatment of the posterior maxilla.