Fighting over when owners of limited partnerships must pay self-employment taxes has lasted nearly five decades. This struggle is attributable to several things, including the absence of applicable regulations, rapid evolution of business entities, and more. Uncertainty has caused taxpayers to claim disparate tax positions and triggered several big-dollar cases. This article, just one in a series, explores the relevant rules, a long list of arguments advanced by taxpayers and the IRS in two pending cases, and the recent Tax Court ruling that it must apply a “functional test” to determine whether a partner in a limited partnership meets the relevant exemption.