The formation process is the final step in lithium-ion cell manufacturing and has a major impact on the performance, the cyclelife and the safety of the cells. To support the formation of a highly permeable and stable solid electrolyte interphase (SEI), the formation procedure must be well defined. The purpose of this paper is to outline the importance of the constant voltage (CV) charging step during the formation process of lithium-ion cells. Therefore, Li(Ni1/3 Co1/3 Mn1/3)02/graphite based lithium-ion cells are charged with and without CV charging step during the formation process and their aging behavior is compared. For a deeper understanding of the aging mechanism, a differential voltage analysis (DVA) as well as several post-mortem analyses, e.g. SEM/EDS analysis and areal capacity determination by half-cell measurements, are performed. A strong correlation of the CV charging step during the formation process and the aging behavior of the cells is concluded. It is concluded that the omission of the CV charging step provokes the formation of an inhomogeneous and porous SEI, leading to accelerated loss of cycleable lithium and graphite anode degradation.