Observing behavior is an operant behavior that has the production of discriminative stimuli as consequence. This presentation will review the main course of the scientific production in this field, from early investigations up to some of the most recent works. To analyze this quite numerous and diverse literature, the speaker will identify and describe the basic observing-response procedures that employed humans as participants, especially children and adults. The maintenance of observing behavior by discriminative stimulus of extinction (that is, S-) compared to discriminative stimulus of reinforcement (that is, S+) will be addressed and two sets of empirical works involving adults and children with typical and atypical development will be presented. The first work will present the current advances in the study of observing behavior by tracking the participants' eye movements. The second work will focus on the role of observing responses in conditional discrimination processes that lead to the formation of equivalence classes. Based on these recent findings, the speaker will discuss how discrimination is established as the product of the main three-term contingency interlocked with the observing-response contingency.