Fault tolerance is necessary for reliable quantum computing. While there have been experimental tests of quantum error-correcting codes, testing fault-tolerance remains a major challenge, mainly due to the substantial overhead. I will describe a qubit-efficient paradigm for fault-tolerance circuit design, termed the "flag method." Traditional approaches achieve fault-tolerance by preventing errors from spreading badly. In the flag method, "flag qubits" are added to catch the faults that can lead to correlated errors on the data. Provided the flag gadgets are carefully designed, the potential correlated errors can be diagnosed by their syndromes, and accordingly be remedied. I will illustrate the flag idea with several applications and extensions, which are of general theoretical interest as well as practical importance.