Detecting eyes in images is fundamental for many computer vision applications including face detection, face recognition, and human-computer interaction. Most existing methods are designed and tested on datasets acquired under controlled lab settings (e.g., fixed scale, known poses, clean background, etc.), leaving their performance to be further examined on real-world, uncontrolled images, such as on-line images. This paper presents an effort on developing a fast and accurate eye detector for on-line images for which the acquisition condition is unknown and varies from one image to another, resulting in unpredictable background and variable scales for the eyes/faces. The key idea is to develop a scale-adaptive EigenEye approach, which employs an approximate scale estimated from face detection to modulate the pre-trained EigenEye basis in searching for the best match in a test image. The effort also includes building a 2845-image dataset with accurately-annotated eye locations and size, which will be made public to the community for future comparative study. Evaluation using this dataset, with comparison with a few leading state-of-the-art approaches, demonstrates the advantages of the proposed method.