Batteries with metallic lithium anodes offer improved volumetric and gravimetric energy densities; therefore, future batteries including the promising lithium–sulfur and lithium–air systems would benefit from them. The electrodeposition of lithium metal – which is an unwanted incident in lithium ion systems – often results in fine filaments or moss, called dendritic lithium, which leads to strong capacity fading and the danger of internal short circuiting. To study the mechanisms of dendritic growth and the behavior during lithium dissolution, lithium deposits have been observed in situ in 1 M LiPF6 in EC:DMC by light microscopy. The high resolution optical microscopy provided information on the growth and electrodissolution of single lithium filaments. The growth areas could be identified in detail: The lithium wires can grow either from the substrate–lithium interface, at kinks or in a region at or close to the tip. Based on these observations, we suggest a growth model for lithium filaments predicated on defect-based insertion of lithium at the aforementioned locations. This type of growth is not compatible with previous models of dendritic growth, for example, it is hardly influenced by electric fields at the tip and does not depend on the direction of the electric field.