Ranking is an important concept in modelling our preferences. We rank hotels by their quality using one star to five stars; we rank baseball teams by their records using pairwise competitions; we rank job applicants by their ability using ordered scores. In machine learning, the ranking concept corresponds to a rich family of important problems, which lend themselves to a wide range of applications from social science to behavioural science to information retrieval. For instance, in a Web search system, we want the machines to automatically rank/order the results of our query based on relevance; in an online shopping system, we want the machines to automatically rank/rate the products based on user evaluations; in a music playing system, we want the machines to automatically rank/recommend the songs based on our personal tastes.