The goal of clinical research is to improve clinical practice. In progressive neurodegenerative conditions without any disease-slowing therapies, this will result in eventual approval of a first disease-modifying treatment. Clinical trials will still be needed to discover treatments that are more effective, safer, or more convenient. This will generate controversies over how to design these trials; specifically, controversies about the use of a placebo control. We consider ethical guidance for these studies with attention to 3 designs: placebo-controlled trials in the absence of the new drug, placebo-controlled trials with the approved drug as background therapy, and trials with the new drug as an active control. To understand the practical implications of these designs, we examine experiences in drug development in multiple sclerosis. We conclude by contemplating the future of clinical trials in Alzheimer disease.