This is a common-sense perspective on how the interaction between languages influences the development of each. For many years, languages were seen as isolated, and historical linguistics focused on internal patterns to explain language change. The authors of this book explain how interference from another language can influence a language, in varying degrees of contact. I appreciated all of the examples from diverse contact situations. It was amazing to me to read through the section at the end, by far the most in-depth example, of the history of the English language. This is a must for anyone interested in historical linguistics. It cleared up a lot for me. I'd also like to see where current research is in this area, since this book was written in the early 1990s based on research mostly from the 70s and 80s.