Li metal dendrites, which can form on the anode of Li-ion batteries during charging, not only accelerate their aging but may also pose a safety hazard when causing a short-circuit within the battery. Therefore, a fundamental understanding of the mechanisms governing the early stages of Li plating, the progression into dendrites, and the formation of dead Li, is imperative. Here, we employ operando electrochemical liquid cell scanning transmission electron microscopy (ec-LC-STEM) to monitor, in real-time, nanoscale processes occurring at the anode-electrolyte interface of a Li-ion battery during charge/discharge. Our results indicate that Li metal dendrites nucleate as spherical Li nanoparticles beneath the solid electrolyte interphase (SEI) and subsequently grow until dendritic Li metal is formed. During discharge, Li dendrites undergo incomplete dissolution, leading to the formation of dead Li. Interestingly, the SEI layers play a pivotal role in both the growth and the dissolution processes. Our findings reveal that the growth of Li dendrites is a multi-step process: (i) nucleation, (ii) root growth, and (iii) tip growth. We elucidate that the formation of dead Li is associated with the morphology of the initially developed dendrites and the structure of the SEI layer. The thinning of the inhomogeneously thick Li whiskers leads to the contraction of the root before the tip, ultimately resulting in the creation of electrically isolated Li metal. This work sheds light on dendritic Li growth as well as on the formation of dead Li and provides significant insights for future electrode designs.