A great deal of progress has been made in the management of Alzheimer's disease and other dementias over the past 25 years. Much remains to be achieved, however. This article discusses some of the issues surrounding study design. In the absence of an accepted biological marker of progression, it is unlikely that a novel study design, such as randomized start or withdrawal, in itself could provide convincing evidence of disease modification. Biological markers will also be crucial in the development of therapies aimed at specific processes, and of immunotherapies.