A microelectronic device comprises pillar structures comprising semiconductive material, contact structures in physical contact with upper portions of the pillar structures, and conductive structures over and in physical contact with the contact structures. Each of the conductive structures comprises a lower portion having a first horizontal width, an upper portion vertically overlying the lower portion and having a second horizontal width greater than the first horizontal width, and an additional portion vertically interposed between the lower portion and the upper portion and having arcuate horizontal boundaries defining additional horizontal widths varying from the first horizontal width proximate the lower portion to a relatively larger horizontal width proximate the upper portion. Memory devices, electronic systems, and methods of forming microelectronic devices are also described.