AbstractA summary is given of recent work on the observation of dislocations in polyethylene crystals and the results are discussed in terms of the crystal‐defect model of polymer structure. Both inter‐ and intralamellar dislocations have been observed and their Burger's vectors determined. Most dislocations were observed to be imperfect or partials, indicating that there exist low energy stacking faults. Annealing single crystal brought about approximately a thousandfold increase in the dislocation density which indicates that dislocations are involved in the process of thickening. The observation of dislocation networks in the folds between lamella prove that these folds are sufficiently regular to pack crystallographically. Observed interactions between inter‐ and intra‐lamellar dislocations show that the interlamellar dislocations are glissile and therefore interlamellar slip can occur by a dislocation motion. These observations lend considerable support to the crystal‐defect model of polymer structure. However, it is pointed out that this model does not conflict seriously with tlie statistical equilibrium theory of polymer crystallization. Dislocations are non‐equilibrium defects and hence would not exist under true equilibrium