The pedunculopontine tegmental nucleus (PPTg) is involved in Parkinson's disease and has become a therapeutic target. However, its normal functions are uncertain: are they motor, sensory or integrative? This position paper reviews PPTg structure and considers experiments designed to understand its behavioural functions. The PPTg is part of the corticostriatal architecture and, consistent with this, a core deficit following lesion is the inability to properly establish action–outcome associations. Understanding normal PPTg structure and function will provide insight into the role it has in Parkinson's disease and related disorders, and will benefit the development of surgical treatments aimed here.