This book is dedicated to Professor David Crolla who passed away unexpectedly while the book was in production. David led an unusually full and productive life both in work and play, achieving great success and popularity. David was a leading researcher, an inspiring teacher, an excellent supervisor of research postgraduates and a friend to many. David's energy, enthusiasm and irrepressible humour made a lasting impression on me and everyone who knew him. He is sorely missed and his essential contribution to the publication of this book will always be remembered.