A memory device includes a substrate and a stack including word lines and interlayer insulating patterns alternatingly stacked on the substrate. The word lines extend in a first direction. Semiconductor patterns cross the word lines and have longitudinal axes parallel to a second direction. The semiconductor patterns are spaced apart from each other in the first direction and a third direction. Bit lines extend in the third direction and are spaced apart from each other in the first direction. Each of the bit lines contacts first side surfaces of the semiconductor patterns spaced apart from each other in the third direction. Data storage elements, which are respectively provided between vertically adjacent interlayer insulating patterns and contact second side surfaces opposite to the first side surfaces, and substrate impurity layers provided in portions of the substrate at both sides of the stack, are included.