The lipid and protein tyrosine phosphatase, PTEN, is one of the most frequently mutated tumor suppressors in human cancers and is essential for regulating the oncogenic prourvival PI3K/AKT signaling pathway. Because of its diverse physiological functions, PTEN has attracted great interest from researchers in multiple research fields. The functional diversity of PTEN demands a collection of delicate regulatory mechanisms, including transcriptional control and posttranslational mechanisms that include ubiquitination. Addition of ubiquitin to PTEN can have several effects on PTEN function, potentially regulating its stability, localization, and activity. In cell and in vitro ubiquitination assays are employed to study the ubiquitination-mediated regulation of PTEN. However, PTEN ubiquitination assays are challenging to perform and the data published from these assays has been of mixed quality. Here we describe protocols to detect PTEN ubiquitination in cultured cells expressing epitope tagged ubiquitin (in cell PTEN ubiquitination assay) and also using purified proteins (in vitro PTEN ubiquitination assay).