Magnetic resonance imaging (MRI) has become essential for the study of dementia. It is a supporting tool for the diagnosis of most neurodegenerative diseases and has shed light on many important aspects of disease etiology and progression. In Alzheimer’s disease and frontotemporal lobar degeneration in particular, it has helped to describe brain networks exhibiting selective vulnerability to neurodegeneration and facilitated the characterization of heterogeneity between clinical and genetic subtypes. MRI is also important for assessing vascular pathology and prion disease. Finally, most MRI modalities capture changes occurring up to decades prior to symptom onset, enabling early disease diagnosis and even prevention. Here,the main MRI techniques used to assess gray matter atrophy, among others, are described. We review recent studies in the different neurodegenerative diseases and describe the most common methodologies used, from visual rating scales to automated morphometry algorithms. Finally, we highlight progress in the theoretical modeling of neurodegenerative diseases and discuss more applied uses of MRI.