Examples relating to calibrating an estimated gaze location are disclosed. One example method comprises monitoring the estimated gaze location of a viewer using gaze tracking data from a gaze tracking system. Image data for display via a display device is received, the image data comprising at least one target visual and target visual metadata that identifies the at least one target visual. The target visual metadata is used to identify a target location of the at least one target visual. The estimated gaze location of the viewer is monitored and a probability that the viewer is gazing at the target location is estimated. The gaze tracking system is calibrated using the probability, the estimated gaze location and the target location to generate an updated estimated gaze location of the viewer.