This thesis is based on the following publications that have arisen as part the thesis work. Full details of these papers below can be found in the bibliography. The models presented in Chapter 4 were introduced in [216, 220] and this chapter is further based on . The concept-based language models in Chapter 5 were introduced in  and further built upon in [207, 208, 212, 215, 221]. The work on linking search engine queries to structured knowledge sources in Chapter 6 was published in  and expanded in . The work in Chapter 7 is based on material published in . Finally, material from a number of other papers, including [26, 126, 138, 210, 217, 218, 227, 317, 338], have been incorporated at various points in the thesis.