The concept of mild cognitive impairment (MCI) has been introduced to describe older individuals who cognitively lie between normal ageing and dementia. Nowadays, there is a particular interest in MCI because this syndrome is thought to be a transitional stage to Alzheimer's disease (AD) that may define a window for effective therapeutic interventions. However, not all patients with MCI will go on to develop AD. Imaging offers an extraordinary opportunity to study MCI. We will review key findings of brain imaging studies in MCI, including structural brain changes studied with MRI, white matter changes with diffusion tensor imaging and altered brain activity and blood flow studied with various imaging modalities, such as positron emission tomography, single-photon emission computed tomography and arterial spin labelling MRI, a non-invasive approach to measure cerebral blood flow. The strength and limitations of each modality for diagnosis of MCI, prediction of MCI outcome and assessment of drug efficacy will be discussed. © 2007 The British Institute of Radiology.