Magnetic resonance imaging (MRI) is a non-invasive imaging method used routinely in the clinic, in clinical research, and in preclinical research. In this lecture, the theoretical basis of MRI will be presented. Topics covered will include NMR, T1-relaxation, T2-relaxation, the links between NMR and MRI, the concept of a pulse sequence, and rudimentary image reconstruction. Current MRI methods, with a special emphasis on functional magnetic resonance imaging (fMRI), will also be presented. In fMRI, the MRI pulse sequence is manipulated such that rapidly acquired, serial MRI images are sensitized to blood oxygenation in the brain. The foundational tenant that areas of brain activation correlate with increased blood flow allows fMRI to be used to monitor brain activity in response to subject tasks. Resting state fMRI, that is fMRI in the absence of a subject task, will also be reviewed.