Brain slices were introduced as a standard preparation for neurophysiological experiments some 20 years ago. A drawback of this preparation compared with cell culture has been the difficulty to visualize individual neurones in standard thick slices. This problem has been overcome by the use of infrared videomicroscopy. Neurones in slices can now be visualized in great detail, and neuronal processes can be patch-clamped under direct visual control. Infrared video-microscopy has also been applied successfully to other fields of neuroscience such as neuronal development and neurotoxicity. A further development of infrared videomicroscopy enables one to visualize the spread of excitation in slices making the technique a tool for the direct investigation of neuronal function.