Many degenerative diseases are fundamentally associated with aging and the accumulation of misfolded proteins as amyloid fibrils. Although such diseases are associated with different proteins, they share several pathological features. These similarities might be due to underlying commonalities in the pathway of aggregation and the structures of the various aggregation products. Because protein misfolding is thought to be central to the pathological state, it is essential to be able to distinguish such pathological states from native and non-pathological states, especially in vivo or in complex mixtures. Conformation-dependent antibodies that specifically recognize misfolded proteins are proving to be useful tools for examining the mechanisms of amyloid formation and for clarifying the roles of various misfolded states in pathogenesis. The common structures and mechanisms hold promise for the development of broad-spectrum drugs and vaccines that will be effective for the treatment of many of these diseases.