Historically informed performance (also referred to as period performance, authentic performance, or HIP) is an approach to the performance of Western music and theater. Within this approach, the performance adheres to state-of-the-art knowledge of the aesthetic criteria of the period in which the music or theatre work was conceived. Whenever this knowledge conflicts with current aesthetic criteria, the option of re-training the listener/viewer, as opposed to adapting the work, is normally followed. Music is usually played on instruments corresponding to the period of the piece being played, such as period instruments for early music.