The notion of using the concept of the immune system as a guide to developing long-term sustainable policies for managing ecosystems is appealing. The immune systems we see today have stood the test of time. They are clearly successful systems for dealing with invasions of dangerous and unwanted microorganisms. Janssen (2001) offers an interpretation of the basic elements of an immune system. He also explores the use of this structure to develop an understanding of how to deal with invasive organisms in ecosystems and, more generally, how to design institutions that will promote long-term sustainability. He extends the notion of invasions beyond the usual one of nonhuman species to include other humans, technological innovations, and even cultural invasions (religions and Big Macs), but then restricts the inquiry to ecosystem management, focusing on the institutions that give rise to successful management practices and on the management of biological invasions. An obvious and immediate concern when attempting to use the immune system of an organism as a model for managing an ecosystem is the essential difference between the two: organisms are evolved, exquisitely integrated, homeostatic entities whose immune systems are managed subconsciously, whereas ecosystems are open, selected combinations of interacting species that are managed very consciously by humans. We need to explore the consequences of these differences before putting the analogy into practice. (Brian Walker)