Sperm are exquisitely sensitive: they can detect a single molecule of chemoattractant that evokes an electrical signal followed by a behavioral motor response. I will outline the signalling pathway that endows sperm with such ultrasensitivity. Moreover, I will discuss how this pathway controls precise navigation in 2D and 3D gradients of chemoattractants provided by the egg. Finally, the common motifs of ultrasensitive signalling in photoreceptors and sperm will be discussed.