The paper introduces the concept of partial synchrony in distributed systems, which lies between synchronous and asynchronous systems. In a synchronous system, there are known upper bounds on message transmission time and processor speed differences, while in an asynchronous system, no such bounds exist. The authors consider two versions of partial synchrony: one where the bounds are unknown but must hold for some time, and another where the bounds are known but only start holding from an unknown time. They design fault-tolerant consensus protocols for various fault models and partial synchrony scenarios, achieving optimal resiliency in most cases. The protocols use distributed clocks to allow partially synchronous processors to agree on a common notion of time. The paper also presents lower bounds showing the optimality of the protocols in terms of the number of faults tolerated. The results are applicable to both synchronous and asynchronous communication systems, with the protocols being adapted to the specific model of partial synchrony.The paper introduces the concept of partial synchrony in distributed systems, which lies between synchronous and asynchronous systems. In a synchronous system, there are known upper bounds on message transmission time and processor speed differences, while in an asynchronous system, no such bounds exist. The authors consider two versions of partial synchrony: one where the bounds are unknown but must hold for some time, and another where the bounds are known but only start holding from an unknown time. They design fault-tolerant consensus protocols for various fault models and partial synchrony scenarios, achieving optimal resiliency in most cases. The protocols use distributed clocks to allow partially synchronous processors to agree on a common notion of time. The paper also presents lower bounds showing the optimality of the protocols in terms of the number of faults tolerated. The results are applicable to both synchronous and asynchronous communication systems, with the protocols being adapted to the specific model of partial synchrony.