Abstract: It is well known that an external stimulation may change the spectrum of the electroencephalogram (EEG).  These changes may be either time- or phase-locked to the stimulation.  An example of this latter occurs in the Evoke Potential exam, where intermittent, periodic stimulation is used in order to elicit a cortical response.  Since the responses are small in comparison with the ongoing EEG, averaging techniques are often required in order to reveal them.  Thus the stimuli are repeated and the corresponding epochs are averaged in order to enhance the EEG component that is accounted for by the stimulation. Spectral changes in EEG that are not phase synchronized with the stimuli, but lead to time-locked changes in powerspectra, cannot be obtained through the averaging techniques used for estimating the evoked potential.  Those changes occur during the time the subject undergoes external stimulation or is asked to carry out a certain task.  They are commonly referred as Event-Related Synchronization and Desynchronization (ERS/ERD), depending on whether there is an increase or decrease in a given frequency band. In this presentation methodologies developed at UFRJ for quantifying both time and phase locked spectral changes in the EEG during sensory stimulation is are described.