Expectancy has long been of interest to psychologists and recently has become the focus of research in musical cognition. Four experiments are reported that investigated the formation of expectancies in musically trained listeners and performers. Experiments 1 and 2 examined the factors underlying the formation of melodic and harmonic expectancies, respectively. Both experiments found evidence for the psychological reality of constructs derived from the music-theoretic literature in expectancy formation. Experiment 3 investigated the generation of expectancies for a full musical context (one containing simultaneous melodic and harmonic information) and found that melody and harmony were perceptually independent, such that they combined additively in expectancy formation for a full musical context. Experiment 4 provided a convergent operation for the earlier studies by having skilled pianists perform their expectations for the same passages. These productions strongly correlated with the perceptual expectancies of Experiments 1-3. Taken together, these studies provide evidence for the existence of musical expectancy, as well as delineating some of the factors affecting its formation.