The Rho family of small GTPases plays a key role in the dynamic regulation of the actin cytoskeleton that underlies various important cellular functions such as shape changes, migration, and polarity. We found that Smurf1, a HECT domain E3 ubiquitin ligase, could specifically target RhoA but not Cdc42 or Rac1 for degradation. Smurf1 interacts with the dominant inactive form of RhoA, RhoA N19, which binds constitutively to guanine nucleotide exchange factors (GEFs) in vivo. Smurf1 also interacts directly with either nucleotide-free or GDP-bound RhoA in vitro; however, loading with GTPgammaS inhibits the interaction. RhoA is ubiquitinated by wild-type Smurf1 but not the catalytic mutant of Smurf1 (C699A) in vivo and in vitro, indicating that RhoA is a direct substrate of Smurf1. In this chapter, we summarize the systems and methods used in the analyses of Smurf1-regulated RhoA ubiquitination and degradation.