Interactions between boats and cetaceans influence the behaviour of animals. Cetaceans will tend to evade boat interactions and these evasion tactics will lead to altered activity budgets (Williams et al., 2002, 2006; Nowacek et al., 2001, Lusseau, 2003, 2004). In some instances, those influences can have severe impacts for individuals and their populations (Lusseau, 2004, 2005a; Williams et al., 2006; Lusseau et al., 2006; Bejder et al., 2006). However, we currently have no way to determine a priori when boat-interaction effects will lead to biological impacts and when those will have population consequences. This lack of predictive power means that we lack the scientific foundations to manage the current rapid growth of the whale-watching industry and therefore the industry's sustainability is being questioned (Corkeron, 2004; Higham & Lusseau, 2007). In this chapter, I develop a conceptual understanding of how the natural constraints that populations face can be critical factors increasing the propensity for population consequences of whale-watching disturbances. This conceptualization is leading to the development of a number of predictions that I encourage the scientific community to test. Each of these hypotheses is highlighted in their relevant section and I also bring them together in the conclusion.