In the last three decades, information sources, especially the internet, are growing up more and more rapidly. Thus, people feel very comfortable to find any information they want from these sources. Experiences and observations show that when an agent wants to get any information, it usually gets a large amount from various online sources. However, in contrast to the easiness of obtaining the information, processing the information from heterogeneous sources becomes a difficult job. The main problem is that these pieces of information from different sources are usually inconsistent, and even the information itself contains uncertainty. Therefore, it becomes an important issue to utilize these pieces of information in a consistent and rational manner. This book concentrates on using merging and revision operations to deal with these situations.