A method of forming a microelectronic device comprises forming a microelectronic device structure comprising memory cells, digit lines, word lines, and isolation material. Contact structures are formed to extend through the isolation material. Some of the contact structures are coupled to some of the digit lines, and some other of the contact structures are coupled to some of the word lines. Air gaps are formed to be interposed between the contact structures and the isolation material. An additional microelectronic device structure comprising control logic devices and additional isolation material is formed. After forming the air gaps, the additional microelectronic device structure is attached to the microelectronic device structure. Additional contact structures are formed to extend through the additional isolation material and to the contact structures. The additional contact structures are in electrical communication with the control logic devices. Microelectronic devices, electronic systems, and additional methods are also described.