The homography induced by the plane at infinity between two images, namely the infinite homography, plays a very important role in 3D computer vision since many vision problems could be substantially simplified by knowing it. Unlike homographies induced by ordinary planes which can usually be determined by correspondences of image points, the infinite homography must be determined indirectly since no real physical points lie on the plane at infinity. In this paper, how to determine the infinite homography through scene parallel planes is studied, and the following two conclusions are proved: (1) If only a pair of parallel planes is present in the scene, the infinite homography can be obtained by solving a 4th order polynomial, and at maximum, four possible solutions exist. (2) If at least two pairs of parallel planes exist in the scene, and if planes in different pairs are not parallel, then the infinite homograpgy can be linearly and uniquely determined. In addition, a geometric interpretation to the above results, and some practical algorithms are also provided. The proposed results in the paper are of interests in camera self-calibration and image based 3D reconstruction under both theoretical and practical standpoints.