Following the definition of perception as pick-up or detection of information (Gibson, 1986; Jacobs and Michaels, 2002), in the ecological approach attention is viewed as the control of detection of information (Michaels and Carello, 1981). The information in the perceptual arrays surrounding observers moving around in their natural environment is taken to be inexhaustibly rich. Because observers cannot pick up all the available information at once they have to be selective. Gibson (1966) called this procedure by which inputs are selected attention (Michaels and Carello, 1981). This is not to say that attention, or the control of detection of information, is always optimal from the start. Findings with respect to perceptual learning (e.g., Jacobs, 2001; Jacobs and Michaels, 2002) indicate that novices often initially rely on non-specifying variables, as they still have to learn to attend to the more useful (i.e., specifying) sources of information. Specifying information sources are specific to the to-be-perceived properties of the environment. This means that detecting a certain information source that specifies a property of interest in the environment allows the observer to make reliable judgments about this property (Beek et al., 2003). A non-specifying information source might be related to the to-be-perceived property, but it is not specific to this property as its value does not under all circumstances reliably predict the value of this to-be-perceived environmental property (ibid.). The present review is concerned with an exploration of information sources and the education of attention in judging offside in football. Furthermore, ways are considered to improve offside judgments that follow from this discussion on information use by assistant referees.