Dislocations in LiCoO2 were observed by transmission electron microscopy, and their Burgers vectors were determined by analysis of diffraction contrast in tilting experiments. The configuration of all dislocations indicates that they are glissile, and dislocation configurations were found that are indicative of active slip planes. Perfect dislocations of a/3 [11 (2) over bar0] type Burgers vectors were observed on {0001} habit planes. These perfect dislocations sometimes dissociate into Shockley partial dislocations with a/3[10 (1) over bar0] type Burgers vectors. Glide of these partial dislocations can account for the sequence of crystal structures O3, H1-3, O1 that occur with the delithiation of LiCoO2. The presence of glissile dislocations also suggests possible damage mechanisms during cycling. (C) 2002 The Electrochemical Society.