A method of forming a microelectronic device comprises forming a microelectronic device structure comprising a first control logic region comprising first control logic devices, and a first memory array region vertically overlying the first control logic region and comprising an array of vertically extending strings of memory cells. An additional microelectronic device structure comprising a semiconductive material is attached to an upper surface of the microelectronic device structure. A portion of the semiconductive material is removed. A second control logic region is formed over the first memory array region. The second control logic region comprises second control logic devices and a remaining portion of the semiconductive material. A second memory array region is formed over the second control logic region. The second memory array region comprises an array of resistance variable memory cells. Microelectronic devices, memory devices, and electronic systems are also described.