This study is a randomized, placebo-controlled 6-month study designed as an adequate and well-controlled trial to demonstrate the safety and efficacy of Dimebon in the treatment of patients with mild-to-moderate AD. Dimebon is an investigational drug for Alzheimer's disease. The target of Dimebon's mechanism of action are the mitochondria (a cell's primary source of energy).The Connection Study is the second of two pivotal studies evaluating the effect of Dimebon. It is a 6-month study enrolling 525 patients in the United States, Europe, and South America. All patients completing the 6-month study will be eligible to receive Dimebon in an open-label extension trial.