This paper presents a new framework for seamless video editing in the gradient domain. The spatio-temporal gradient fields of target videos are modified or mixed to generate a new gradient field, which is usually not integrable. We propose a 3D video integration algorithm, which finds a potential function, whose gradient field is closest to the resulting gradient field in the sense of least squares. The video is reconstructed by solving a 3D Poisson equation. We use a fast and accurate 3D discrete Poisson solver using diagonal multigrids. A set of gradient operators are defined for user interaction. The resulting video has temporal coherency and no artifacts. We evaluate our algorithm using a variety of examples.