The human brain contains approximately 1012 to 1013 cells. About 1011 cells out of this pool can be classified as nerve cells or neurons. Although representing only 2–10 % of all brain cells, it is widely accepted that the neurons and their mutual synaptic connections form the anatomical subsfrate for die powerful computational abilities of the brain. The remaining cells, many of which are called glial cells, are devoted to important auxiliary tasks: They ensure optimal conditions for neuronal signal transmission by preserving important ionic concentration levels and insulating neurons from each other, they accelerate neural signals by myelinization, they shield the brain against toxic substances in the blood by forming the brain-blood barrier and they contribute to the nutrition of neurons. In chapter 4, we will discuss a potential key role of glial cells for the optical measurement of nerve cell activity. Nevertheless, the activities of glial cells are thought to be only of secondary importance for understanding the principles of neuronal signal processing.