In this chapter, I compare the role of the creditor’s (promisee’s) conduct in contractual relationships in US and European legal systems. Different approaches to comparative negligence and mitigation are first considered, and then a more general analysis of doctrines dealing with the creditor’s position in the contractual relationship and the role of cooperation is carried out. In this area, legal systems display significant divergences - partly rooted in their historical antecedents, and partly related to different concepts of contracts and contractual relationships. Continental European systems (with significant differences between Germany and France) recognize a strong role for comparative negligence and the duty to cooperate, while common law jurisdictions (with important differences between England and the US) limit the scope of comparative negligence and the duty to cooperate whilst attributing a wider role to the duty to mitigate In this chapter I will show that the great divergence concerning the rule of comparative negligence in contract law between England and the US on the one hand, and among European continental systems with the exception of France on the other, needs to be rethought. A wider range of doctrines beyond mitigation should be considered on the ground that they act, at least partially, as functional equivalents to comparative negligence.