In 1999, California enacted domestic partnership legislation for the first time. In its initial stages, registration of a domestic partnership offered few rights and no responsibilities to partners. Subsequent legislation added greater rights and responsibilities to the skeleton of the registry, culminating in the Domestic Partner Rights and Responsibilities Act of 2003, which conferred upon registrants nearly all the benefits and obligations of marriage. Although the 2003 Act took effect on January 1, 2005, the California Legislature amended the Act in the interim, applying community property law back to the date of registration under the 1999 law. In doing so, it effectively created a period of retroactivity of up to five years. This Comment examines the permissibility of retroactively applying the Act to conduct occurring before the legislation was enacted. It ultimately concludes that the retroactive portions of the Act violate the Takings Clause of the U.S. Constitution, as well as the Due Process Clause of the California Constitution. While the Comment aims to expose the constitutional infirmity of the Act's retroactivity, its ultimate goal is more prescriptive: It suggests ways in which other states may avoid the pitfalls of California's statute when tailoring their own domestic partnership legislation.