Negative feedback is one of the most difficult topics in physiology, possibly because descrip­tive thinking is useless. Instead, to analyse single constituents quantitatively or semiquantitatively, causal thinking is mandatory. By using analogue simulation, the author tries to show the basic properties of negative feedback. The characteristics of an ideal negative feedback – operat­ing without any appreciable delay – are shown. Then the characteristics of a real negative feedback – including a distinct delay – are shown. Finally, the role of negative feedback in analogue simulation of pulmonary ventilation is described.