As discussed in Chapter 2, in most sentiment analysis applications, one needs to study opinions from many people; due to the subjective nature of opinions, looking at only the opinion from a single person is usually not sufficient. To understand a large number of opinions, some form of summary is necessary. Definition 2.10 in Chapter 2 defined a structured opinion summary called aspect-based summary, also known as feature-based summary in the reports by Hu and Liu (2004) and Liu et al. (2005). Much of the opinion summarization research is based on this definition. This form of summary is also widely used in industry. For example, both Microsoft Bing and Google Product Search use aspect-based summary in their opinion analysis systems.