Why is vaccination against infectious diseases less effective in older patients than in younger ones? Why do the elderly suffer from more frequent and severe infectious episodes than the young? The answer to both these questions is immunosenescence--the poorly defined changes that occur in the immune system as a result of the disrupted performance of multiple components of immunity. Presentations from a recent workshop, which are summarized here, examined these questions and provided some insights from the perspective of improving vaccination strategies among the elderly.