Virtual Reality (VR) video is emerging as a new art form. Viewing VR video requires wearing the VR headset to fully experience the immersive surrounding of the content. However, the novel viewing experience of VR video creates new challenges and requirements for conventional video authoring tools, which were designed mainly for working with normal video on a desktop display. Designing effective authoring tools for VR video requires intuitive video interfaces specific to VR. This dissertation develops new workflows and systems that enable filmmakers to create and improve VR video while fully immersed in a VR headset. We introduce a series of authoring tools that enables filmmakers to work with video in VR: 1) Vremiere, an in-headset video editing application that enables editors to edit VR video entirely in the headset, 2) CollaVR, a networked system that enables multiple users to collaborate and review video together in VR, and 3) a set of techniques to assist filmmakers in managing and accessing interfaces in stereoscopic VR video without suffering depth conflicts. The design of these applications is grounded in existing practices and principles learned in interviews with VR professionals. A series of studies is conducted to evaluate these systems, which demonstrate the potential of in-headset video authoring.