This document discusses image retrieval techniques for content-based image retrieval systems. It begins with an introduction to the growth of digital image collections and the need for large-scale image retrieval systems. It then reviews different features used for image retrieval, such as color histograms, color moments, color coherence vectors, and discrete wavelet transforms. Edge features and corner features are also discussed. The document concludes that using only one feature type such as color or texture is not sufficient, and the best approach is to extract multiple high-quality features and combine them for image retrieval.