The parts of biological systems are different from the parts of physical and chemical systems – they are functional. For example, the sun might cause the effect of warming the earth, but we do not say that this is its function. This is just something that the sun does; it is not an explanation of why the sun exists. By contrast, the heart has the function of pumping blood. This is both an explanation of what the heart does and why it exists. In this dissertation, I explore the functional nature of biological systems by adopting the organisational account of functions. The organisational account claims that biological traits have a function in lieu of the contribution they make to maintain a system they are part of and on which their existence depends. Following this, I explore how the organisational account can accommodatethe collaborative relations that arise between biological systems.