In this research project, we characterized the n-th order cyclostationarity properties of general types of LPI signals and used the characterizations to investigate and develop methods for both secure communication and interception. For secure communication, this includes means for reduction of strength of' and elimination of, cyclic features that could be exploited by an interceptor. Signal design for LPI communication was considered from the viewpoints of both the communicator's reception task and the interceptor's reception task. For interception, the characterizations were used to propose feature sets and discrimination rules for signal classification and identification. The basic approach to characterizing n-th order cyclostationarity properties of signals was extended and generalized from the recently developed theory of 2nd-order cyclostationarity, in which spectral characterizations play a crucial role.