Determining responsibility for speech is important for two reasons: to address rights to forum access and to identify whether Establishment Clause limits apply. Private speakers may demand rights of access to a public forum, and in such a forum they may articulate their message free from viewpoint restrictions. Private speech, moreover, is not subject to Establishment Clause limits. If the speech is government speech, the Free Speech Clause does not apply, and the government may articulate its message to the exclusion of all other speakers. If the government speech has religious content, it may run afoul of the Establishment Clause. This Article proposes an “effective control” framework to determine Establishment Clause responsibility in cases where public and private actors jointly engage in speech. Between the end-points of purely governmental and purely private speech, it places such speech on a mixed speech continuum. After introducing the framework, this Article demonstrates how the theory of “effective control” functions in a variety of contexts implicating the Establishment Clause, including permanent and temporary displays, prayer in public schools, access to public school property, and legislative prayer. In some instances, discussed as “truly hybrid speech” in this Article, the effective control inquiry fails to identify a unilaterally responsible party. In these limited cases, this Article argues that the speech is sufficiently private for forum access purposes--meaning that the speakers may claim a right to forum access--and at the same time sufficiently governmental for Establishment Clause purposes, potentially creating a secular forum in certain narrowly defined speech contexts.