Organisms are responsive--they respond to stimuli. This is a unique mode of causation that we usually only ascribe to organisms. What does it amount to? In this talk, I propose two candidate theories of responsiveness. The first is a functional pathway theory according to which organisms that are responsive are organisms with a certain kind of physiologically realized functional architecture. The second is a vital-integrative theory, according to which responsiveness is a capacity of whole organisms to integrate their activity with the environment in such a manner that their needs are met. I will explain the two views and their underlying rationales. Finally, I will argue that these two theories attribute different kinds of causal structure to the organism, and say divergent things about how their activity is organized. Adjudicating between these views could help to resolve a deeper, older debate between mechanistic and organicist theories of the organism. Therefore, we should find ways to test these theories of responsiveness.