Magnetoencephalography (MEG) is an invaluable tool to study the dynamics and connectivity of large-scale brain activity and their interactions with the body and the environment in functional and dysfunctional body and brain states. This primer introduces the basic concepts of MEG, discusses its strengths and limitations in comparison to other brain imaging techniques, showcases interesting applications, and projects exciting current trends into the near future, in a way that might more fully exploit the unique capabilities of MEG. This primer by Gross introduces magnetoencephalography (MEG) as a versatile tool to study large-scale brain activity in health and disease. It explains fundamental concepts of MEG and discusses recent and future applications in the field of cognitive neuroscience.