Personalized recommendations are ubiquitous in social network and shopping sites these days. How do they do it? As long as enough user interaction data is available for items e.g., products in shopping sites, a kind of recommendation engine based on what’s known as ' Collaborative Filtering' is not that difficult to build. Since the solution causes a combinatorial explosion, Hadoop can play a critical role in processing massive amount of data in collaborative filtering based solutions. In this presentations, I will cover a Hadoop based recommendation engine implementation using collaborative filtering.