It was a rainy day in December and we were sitting in an office at the Nuclear Physics Center in Lisbon deeply involved in a heated discussion about the opening of this book. Should we follow the standard practice, or should we paint the big picture? True to our main motivation, after hours we finally agreed.

The human fascination for a clear starry sky is timeless. It has been around since the early days of mankind and includes the most diverse cultures. Only in the last century, nuclear physics has started to make a very important contribution to our understanding of these phenomena in the sky. And until the present day, many big questions connected to nuclear reactions remain to be answered. One of the prime examples listed amongst the eleven most important physics questions for our century is this: ‘How and where are the heavy elements produced?’.

Why another book? For decades we have come across colleagues, including experimentalists, who would like to learn more about reactions. Some have become fluent in running reaction codes, but cannot find a book at the right level to learn the theory associated with the calculations they are performing. Probably the largest push toward embarking on the adventure of writing this book came after several years of teaching reaction theory to graduate students. The reference nuclear reaction books have been around for decades, and even though there have been some more recent efforts, nowhere could we find the appropriate level, detail, connection to the present experimental scene, the guiding motivation of astrophysics, and the content consistent with that motivation.