Commonly encountered biological signals that describe the electrical activity of the brain, heart, muscles etc will be introduced in this section. Some of these signals like ECG and electroencephalogram (EEG) are spontaneous activity of the human body while others such as evoked potentials are signals in response to external stimulus, for example the presentation of visual stimuli which results in visual evoked potential (VEP) signals. While some analysis procedures are common (like using filters to extract components in specific frequency range), the different properties of these biological signals do call for the application of widely varying analysis procedures. Hence, it is useful to know the fundamental details of how these signals are generated before studying analysis procedures for extraction of the required information.