It is well known that geometry of contact element is an extremely important factor affecting adhesion. Comparing various biological adhesive systems, we find that mushroom-shaped geometry of contact elements is widely spread in nature. Based on experimental data obtained with an artificial model system implementing a mushroom-shaped geometry of contact element, we discuss the principles responsible for particularly strong adhesion in this type of contact. Finally, we draw a general relationship between functional types of different biological adhesive systems and geometry of contact elements.