Nowadays, computers are capable of storing massive amounts of information interconnected in a large-scale network, the Internet. These characteristics make indispensable tools that can retrieve specific information from this network. One of these key applications is a question answering system. More precisely, query logs have shown that a particular type of question is very prominent: definition questions (e.g., "Who was Martin Luther?" and "What is a solar panel?"). Broadly speaking, the answer to this sort of query comprises an array of pieces of relevant information about the concept being defined (a.k.a. definiendum). Until recently, definition queries remained a largely unexplored area for question answering. Standard technology designed to respond other kinds of queries cannot be straightforwardly applied to this task. This book presents the fundamentals, the state-of-the-art methodologies for dealing with this class of question, along with proposing new strategies in the context of my PhD. work.