The PD (Parkinson's disease) protein LRRK2 (leucine-rich repeat kinase 2) occurs in cells as a highly phosphorylated protein, with the majority of phosphosites clustering in the region between the ankyrin repeat and leucine-rich repeat domains. The observation that several pathogenic variants of LRRK2 display strongly reduced cellular phosphorylation suggests that phosphorylation of LRRK2 is involved in the PD pathological process. Furthermore, treatment of cells with inhibitors of LRRK2 kinase activity, which are currently considered as potential disease-modifying therapeutics for PD, leads to a rapid decrease in the phosphorylation levels of LRRK2. For these reasons, understanding the cellular role and regulation of LRRK2 as a kinase and as a substrate has become the focus of intense investigation. In the present review, we discuss what is currently known about the cellular phosphorylation of LRRK2 and how this relates to its function and dysfunction.