The problem of detecting faults in arbitrarily large unilateral iterative arrays of combinational cells when any number of modules can be faulty and each module can fail in an arbitrary manner has been considered. A simple design criterion has been given which if incorporated makes the array easily diagnosable. Methods of deriving test sets for the modified arrays, which requires rather trivial computations and no complicated algorithms, have been given. The tests obtained are largely independent of the behavior of the individual modules in the array and are efficient. It has also been shown that Maitra cascades, group cascades, and cutpoint cellular arrays are testable without any modification. Methods of obtaining test sets for these special arrays have been given. Arrays of more than two dimensions have also been considered.