Half-life (t1/2) is the oldest but least well understood pharmacokinetic parameter, because most definitions are related to hypothetical 1-compartment body models that don't describe most drugs in humans. Alternatively, terminal half-life (t1/2,z) is utilized as the single defining t1/2 for most drugs. However, accumulation at steady state may be markedly over predicted utilizing t1/2, z. An apparent multiple dosing half-life (t1/2, app) was determined from peak and trough steady-state ratios and found to be significantly less than reported terminal t1/2s for eight orally dosed drugs with t1/2,z values longer than one day. We define a new parameter, “operational multiple dosing half-life” (t1/2, op), as equal to the dosing interval at steady-state where the maximum concentration at steady-state is twice the maximum concentration found for the first dose. We demonstrate for diazepam that the well-accepted concept that t1/2,z representing the great majority of the AUC will govern accumulation can be incorrect. Using oral diazepam, we demonstrate that t1/2, op is remarkably sensitive to the absorption t1/2, even when this absorption t1/2 is much less than t1/2,z, and describe the relevance of this in designing extended release dosage forms. The t1/2, op is compared with previously proposed half-lives for predicting accumulation.