Coverage is a systems parameter describing the effectiveness of self-test procedures chosen for built-in tests (BIT) and other design provisions, such as recoverability, for protection of a system against faults. Classical reliability equations can be substantially modified by the inclusion of the coverage parameter because failures due to insufficient coverage often turn out to be the major cause of system failure. With good coverage, reliability and availability are greatly enhanced, often much more cost-effectively than by the indiscriminant addition of redundancy and/or spares. A general equation has been found useful in quickly evaluating system MTBF when coverage is taken into account. Both Markov chain and non-Markov chain examples are discussed.