This doctoral thesis presents a definition of vulnerability that resolves the problems of former definitions and outlines the moral implications of vulnerability ascriptions to humans and animals. In the first part, I present a philosophical analysis of the conditions of vulnerability ascription and my definition of vulnerability. I argue that the general vulnerability of living beings relies on their having either welfare or agency interests that could potentially be frustrated. I identify the following manifestations of vulnerability: justified or unpreventable harm, unjustified harm, and harmless wrongs. The second part is dedicated to theoretical issues regarding ascriptions of vulnerability to animals. I show that some animals have welfare interests and sometimes agency interests, which leads to a list of prima facie claims of animals that should be justly considered by moral agents. In the third part, I discuss some case scenarios of just and unjust consideration of the claims of animals.