In the past clinical trials in dementia, symptomatic or disease modifying in nature, relied mostly on clinical/neuropsychological endpoints. In the recent past much effort has been put into incorporating imaging, notably MRI, into trials in order to provide a surrogate marker for progression and measure a disease modifying effect, and to provide more insight into the homogeneity, or absence of, the included sample. In the following review an update will be given on the current thinking on how and when MRI should be used in the design of a clinical trial in Alzheimer's disease and mild cognitive impairment, with particular emphasis on trials focusing on disease modification.