Abstract: Although the practices of animal experimentation and intensive rearing involve a considerable amount of animal suffering they continue to be supported. Why is the suffering of animals in these practices so often accepted? This paper will explore some of the reasons given in support of the use of animals for such practices. In particular I will focus on contractarianism as one of the many positions that argues that morally relevant differences between species justify animal experimentation and factory farming. These differences include rationality and moral agency. On this position non-humans are excluded from direct moral concern on the basis that they lack such qualities. I will argue that in order for contractarianism to be coherent it necessarily has to include non-humans in the contract. This has implications for the application of contractarianism to the ethics of factory farming and animal experimentation.