Only quite recently has it become technically possible to record, from elec­ trodes attached to the scalp, the human brain's electrical responses to sensory stimulation. The core of this book is a critical survey of endeavours to use these electrical responses (called evoked potentials) as tools in attempts to discover the ways in which the brain first processes incoming sensory information and then forms internal representations of features ofthe external world. Buttressed by quantities of undeniably sloppy evoked potential research there are some who would dismiss out of hand the recording of evoked potentials as being comparable to 'holding an oscilloscope probe 6 feet in diameter up to a computer and pronouncing from the resultant waveform on the underlying structure and function'; many would contrast the scientific value of recording the activities of single nerve cells with microelectrodes. However, although it is certainly true that the brain may indeed function in a way which would not be susceptible to the approaches of the evoked potential researcher it might equally well resist the stratagems of the single-cell man. If, for example, the neural correlate of some sensation were the state of some large population of nerve cells, then the present-day researcher who records in great detail the activity of one single cell or of a very few cells would be faced with a major problem in trying to see the forest for the trees.