Two sets of arrows that exhibit the Müller-Lyer optical illusion. The set on the bottom shows that all the shafts of the arrows are of the same length.

The Müller-Lyer illusion is an optical illusion consisting of a stylized arrow. When viewers are asked to place a mark on the figure at the midpoint, they invariably place it more towards the "tail" end. It was devised by Franz Carl Müller-Lyer (1857–1916), a German sociologist, in 1889.

A variation of the same illusion (and the most common form in which it is seen today, see figure) consists of a set of arrow-like figures. Straight line segments of equal length comprise the "shafts" of the arrows, while shorter line segments (called the fins) protrude from the ends of the shaft. The fins can point inwards to form an arrow "head" or outwards to form an arrow "tail". The line segment forming the shaft of the arrow with two tails is perceived to be longer than that forming the shaft of the arrow with two heads.