Invasion biology has many hypotheses, but it is largely unknown whether they are backed up by empirical evidence. This book fills that gap by (a) further developing a tool for assessing research hypotheses and (b) applying it to a number of invasion hypotheses, using the hierarchy-of-hypotheses (HoH) approach. Part 1, an overview chapter of invasion biology will be followed by an introduction to the HoH approach and short chapters of science theorists and philosophers that comment on the approach. Part 2 outlines the 30 invasion hypotheses and their interrelationships. Part 3 suggests future directions of invasion research.