Two Parkinson's disease (PD)-associated proteins, the mitochondrial kinase PINK1 and the E3-ubiquitin (Ub) ligase PARKIN, are central to mitochondrial quality control. In this pathway, PINK1 accumulates on defective mitochondria, eliciting the translocation of PARKIN from the cytosol to mediate the clearance of damaged mitochondria via autophagy (mitophagy). Throughout the different stages of mitophagy, post-translational modifications (PTMs) are critical for the regulation of PINK1 and PARKIN activity and function. Indeed, activation and recruitment of PARKIN onto damaged mitochondria involves PINK1-mediated phosphorylation of both PARKIN and Ub. Through a stepwise cascade, PARKIN is converted from an autoinhibited enzyme into an active phospho-Ubdependent E3 ligase. Upon activation, PARKIN ubiquitinates itself in concert with many different mitochondrial substrates. The Ub conjugates attached to these substrates can in turn be phosphorylated by PINK1, which triggers further cycles of PARKIN recruitment and activation. This feed-forward amplification loop regulates both PARKIN activity and mitophagy. However, the precise steps and sequence of PTMs in this cascade are only now being uncovered. For instance, the Ub conjugates assembled by PARKIN consist predominantly of noncanonical K6-linked Ub chains. Moreover, these modifications are reversible and can be disassembled by deubiquitinating enzymes (DUBs), including Ub-specific protease 8 (USP8), USP15, and USP30. However, PINK1-mediated phosphorylation of Ub can impede the activity of these DUBs, adding a new layer of complexity to the regulation of PARKINmediated mitophagy by PTMs. It is therefore evident that further insight into how PTMs regulate the PINK1-PARKIN pathway will be critical for our understanding of mitochondrial quality control.Phosphorylation and ubiquitination are two post-translational modifications (PTMs) that often occur together in the regulation of signaling cascades. Examples of pathways regulated by both include the epidermal growth factor (EGF) receptor and NFκB signaling pathways (Karin and Ben-Neriah 2000;Nguyen et al. 2013). Typically, the addition of a phosphate moiety onto a particular residue of a substrate protein regulates the ability of an E3-ubiquitin (Ub) ligase to attach Ub onto lysine residues within a substrate (Lin et al. 2002;Gallagher et al. 2006;Dou et al. 2012). Considering the prominent cross-talk between these PTMs, it is not surprising that they are also implicated in many disease-associated pathways, including cancer and neurodegenerative diseases such as Parkinson's disease (PD). PD is a progressive neurodegenerative disorder that in most cases occurs sporadically. However, the discovery of genes responsible for rare familial cases of PD has shed light on the pathogenesis of the disease. For instance, recessive loss-of-function mutations in the PARK2 and PARK6 genes encode the E3-Ub ligase PARKIN and the mitochondrially targeted kinase PINK1, respectively. These two proteins act together through an intricate int...