The increasing life expectancy in our society results in a continuously growing number of patients suffering from neurodegenerative disorders, particularly Alzheimer's disease (AD). Apart from the deleterious consequences for patients and their relatives, this issue has also alarming effects on our social systems. These facts have justified increased scientific efforts regarding the identification of basic pathomechanisms of dementia and the development of new treatment options. Increased production of specific proteins and their pathologic aggregation in the brain appears to be a pathomechanism which occurs early in the course of many different neurodegenerative diseases. Among the most well-known of these protein aggregations are amyloid plaques, which arise from the aggregation of the β-amyloid protein. Currently, this amyloid-aggregation pathology is regarded as a key pathology, playing a causal role in the development of AD. Consequently, modern therapy approaches are directed towards this target. Limited access to brain tissue has so far restricted the definite diagnosis of AD to postmortem histopathological assessment of brain tissue. For the same reason, a clear association between extent of amyloid deposition pathology and clinical course of AD has not been established so far. However, particularly with regard to new therapeutic options, a reliable in vivo diagnosis is required. Modern molecular imaging tracers such as [11C]PIB do now open the possibility to visualize amyloid depositions in vivo, using positron emission tomography. This type of "in vivo histopathology" approach allows the characterization of neurodegenerative disorders on the basis of the underlying pathology rather than on their symptomatic appearance. In this manuscript, we will discuss the options of amyloid-plaque imaging regarding early and differential diagnosis of different forms of dementia as well as for patient selection for therapy trials and for objective therapy monitoring.