Computer science, like other mathematical fields, cannot live without a tight relationship with reality. However, such a relationship is, frankly, not very common. This is probably why people so enthusiastically welcome a true meeting of theory and practice. In that sense, the coming together of XML and tree automata theory was a beautiful marriage. Thus I have written this book in the earnest hope that the news of this marriage will be spread and celebrated all over the world!

The book is a summary of my ten years' work. It could not have been realized without my collaborators, Peter Buneman, Giuseppe Castagna, Alain Frisch, Vladimir Gapeyev, Kazuhiro Inaba, Shinya Kawanaka, Hiromasa Kido, Michael Y. Levin, Sebastian Maneth, Makoto Murata, Benjamin C. Pierce, Tadahiro Suda, Jérôme Vouillon, Takeshi Yashiro, and Philip Wadler. In particular, I thank Kazuhiro Inaba and Sebastian Maneth, who made uncountable comments on the draft and thus contributed to a huge improvement of the book. Lastly, I thank my dear wife, Ayako, who gave me the warmest and unceasing encouragement to finish the book.