This paper presents some applications of edge detection to digital image watermarking. It discusses and compares several edge detectors' applications in the selection of embedding positions. These edge detectors are numerous and available, ranging from the simple one like the Sobel operator to sophisticated ones such as the Canny and SUSAN operators. In addition, this paper also presents some applications of edge detection to adaptive image watermarking, in which watermark is generated by detecting the edge information from the original image. The paper also presents an approach to improve the robustness by using edge detectors to extract image feature points.