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Logistics Availability (AL), the probability that a system is not in a spares delay downstate at any instant of time, is a basic element of Operational Availability. Formulas for AL for serial systems of assemblies are derived. Each assembly has exponential times to failure and is supported by a spares inventory that is either periodically resupplied or resupplied as-needed (an order is placed following each failure which requires a spare to restore the system). The formulas depend upon the reliability of the operating assemblies, the specified number of spares in the full-up inventory, the usual resupply times, and the constant spares delay downtime for failures that occur when there are no spares in the inventory. These formulas for AL can be used in well-known integer optimizing processes to select the spares quantities for the site inventory and/or to determine acceptable resupply periods and spares delay downtime, in addition to assessing AL.