The prison mailbox rule applies when a prisoner delivers a habeas petition on behalf of another prisoner to prison authorities for forwarding to the court clerk. Hernandez, a California prisoner, filed a pro se federal habeas petition after his state habeas petitions were denied. The federal habeas petition was dismissed as untimely. The district court refused to apply the prison mailbox rule to calculate the dates of Hernandez’s filings because a prisoner other than Hernandez delivered the petition to prison authorities for mailing to the court clerk. Hernandez appealed. Held: Reversed and remanded. The prison mailbox rule applies to a pro se prisoner’s filing of a California state habeas petition and provides that the petition is filed at the time it is delivered to prison authorities for forwarding to the court clerk. Here, the Ninth Circuit held that this rule also applies when a pro se prisoner gives his habeas petition to a third party prisoner to mail from within the prison. In this situation, the operative date remains the date the petition was delivered to prison authorities, not the date that the petition was delivered to the third party prisoner for delivery. Like a pro se prisoner who delivers a habeas petition to prison authorities himself, a pro se prisoner who gives his petition to another prisoner to mail from within the prison is also unable to monitor the processing of the petition and unable to control the time of delivery of documents to the court once the petition is delivered to prison authorities. The one-year filing deadline will not be circumvented in this situation because the date the petition is delivered to prison officials by the third party prisoner is readily ascertainable and no one will have an opportunity to make substantive revisions after the delivery date.