This is a Phase 2 study designed to evaluate the efficacy and safety of ABT-126 in approximately 260 adults with mild-to-moderate Alzheimer's disease (AD). Subjects will be randomized to one of the four treatment groups (ABT-126, donepezil, or placebo) for a 12-week Treatment Period. Acronyms are listed in the secondary outcome section, below, you will find a list of the acronyms defined: