One of the main reasons why Byzantine fault-tolerant (BFT) systems are currently not widely used lies in their high resource consumption: 3f+1 replicas are required to tolerate only f faults. Recent works have been able to reduce the minimum number of replicas to 2f+1 by relying on trusted subsystems that prevent a faulty replica from making conflicting statements to other replicas without being detected. Nevertheless, having been designed with the focus on fault handling, during normal-case operation these systems still use more resources than actually necessary to make progress in the absence of faults. This paper presents Resource-efficient Byzantine Fault Tolerance (ReBFT), an approach that minimizes the resource usage of a BFT system during normal-case operation by keeping f replicas in a passive mode. In contrast to active replicas, passive replicas neither participate in the agreement protocol nor execute client requests; instead, they are brought up to speed by verified state updates provided by active replicas. In case of suspected or detected faults, passive replicas are activated in a consistent manner. To underline the flexibility of our approach, we apply ReBFT to two existing BFT systems: PBFT and MinBFT.