In order to achieve a 3D, either Euclidean or projective, reconstruction with high precision, one has to consider lens distortion. In almost all work on multiple-views problems in computer vision, a camera is modeled as a pinhole. Lens distortion has usually been corrected off-line. This paper intends to consider lens distortion as an integral part of a camera. We first describe the epipolar geometry between two images with lens distortion. For a point in one image, its corresponding point in the other image should lie on a so-called epipolar curve. We then investigate the possibility of estimating the distortion parameters and the fundamental matrix based on the generalized epipolar constraint. Experimental results with computer simulation show that the distortion parameters can be estimated correctly if the noise in image points is low and the lens distortion is severe. Otherwise, it is better to treat the cameras as being distortion-free.