This chapter explains how to build recommender systems. Recommender systems are the systems that make recommendations to you, based on prior choices exercised by you and by others who have exhibited similar tastes in their choices. When you visit an e-commerce site and look for a specific dress, you start seeing several other dresses which are similar. Or, when you watch a video on YouTube, it starts recommending several other videos which are similar. Similarly, when you read a specific news item, the website starts showing you information related to that same category of items. When you search for a book on amazon.com, it also shows other books in the same genre or by the same author.