Fault Tolerance is a technique used in operating system response to the hardware or software failure and it refers to a system’s ability to allow for failures or malfunctions, and this ability may be provided by software, hardware or a combination of both. To handle faults gracefully, some computer systems have two or more duplicate systems. On the hardware level Fault Tolerance is achieved by duplexing the every hardware component and disks are mirrored. Multiple processors are together and their outputs are compared for correctness. When on the running time when fault is accrued then all the faulty components are determined and taken out of service but the machine continues to function as usual.