Prelithiation is a strategy of increasing importance for high energy density, long cycle life cells. This paper provides a thorough understanding of the implications of prelithiation on cell design and a phenomenological understanding of the behavior of prelithiated negative electrodes in full cells. In a first part, an idealized electrode stack model is derived showing the variation of energy density with prelithiation. Two regimes are identified, the first where prelithiation allows increased energy density by compensating the irreversible capacity of the negative electrode and a second where further prelithiation provides a lithium reservoir to compensate ongoing cycling losses. In a second part coin and cylindrical full cells are used to demonstrate the two regimes. Full coin cells are used to show the impact of the lithium reservoir and the impact of the coulombic efficiency of the negative electrode on the cycle life of a prelithiated cell. Cylindrical 2Ah cells are used to demonstrate the impact of accurate and repeatable roll to roll prelithiation combined with an engineered Si alloy. A cylindrical cell with a prelithiated negative electrode containing 55 wt% Si alloy demonstrated 80% capacity retention at 500 cycles and a coulombic efficiency of over 99.9% up to 700 cycles. (C) The Author(s) 2018. Published by ECS.