Brain states at rest can be defined in two fundamentally different time scales: slowly fluctuating coherent large-scale networks, as observed with functional MRI, and fast switching spatial patterns of global neural activity in sub-second time scale, observable with EEG. EEG studies focusing on the spatial pattern of the global scalp electric field have shown that these fields remain stable for time periods of about 100 ms and then rapidly switch to a new configuration within which they remain stable again. It is hypothesized that these short-lasting states (so-called microstates) represent subsequent time periods during which cooperating brain areas of large-scale networks are activated in a coordinated fashion. Each of these states represents a microstate of cognition so that cognitive processing evolves through a succession of such states. Consequently, changes in mental states by altered levels of consciousness or mental diseases are characterized by changes of the temporal dynamics of the EEG microstates. EEG Microstate analysis has become a standard in the EEG research community, with an exponential increase in publications in all areas of cognitive and clinical neuroscience. This presentation will give an overview of the analytical approach and a summary of the current state of knowledge concerning the functional significance of EEG microstates.