It has now been some years since I completed the first draft of the first edition of this book. In this time, I have learned much from many collaborators and I am grateful to them. During the past few years, Mario Berta, Nilanjana Datta, Saikat Guha, and Andreas Winter have strongly shaped my thinking about quantum information theory, and Mario and Nilanjana in particular have influenced my technical writing style, which is reflected in the new edition of the book. Also, the chance to work with them and others has led me to new research directions in quantum information theory that I never would have imagined on my own.

I am also thankful to Todd Brun, Paul Cuff, Ludovico Lami, Ciara Morgan, and Giannicola Scarpa for using the book as the main text in their graduate courses on quantum information theory and for feedback. One can try as much as possible to avoid typos in a book, but inevitably, they seem to show up in unexpected places. I am grateful to many people for pointing out typos or errors and for suggesting how to fix them, including Todd Brun, Giulio Chiribella, Paul Cuff, Dawei (David) Ding, Will Matthews, Milan Mosonyi, David Reeb, and Marco Tomamichel. I also thank Corsin Pfister for helpful discussions about unique linear extensions of quantum physical evolutions. I am grateful to David Tranah and the editorial staff at Cambridge University Press for their help with publishing the second edition.

So what's new in the second edition? Suffice it to say that every page of the book has been rewritten and there are over 100 pages of new material! I formulated many thoughts about the revision during fall 2013 while teaching a graduate course on quantum information at LSU, and I then formulated many more thoughts and made the actual changes during fall 2015 (when teaching it again). In that regard, I am thankful to both the Department of Physics and Astronomy and the Center for Computation and Technology at LSU for providing a great environment and support. I also thank the graduate students at LSU who gave feedback during and after lectures. There are many little changes throughout that will probably go unnoticed.