A microelectronic device comprises a stack structure comprising alternating conductive structures and insulative structures arranged in tiers, each of the tiers individually comprising a conductive structure and an insulative structure, strings of memory cells vertically extending through the stack structure, the strings of memory cells comprising a channel material vertically extending through the stack structure, another stack structure vertically overlying the stack structure and comprising alternating levels of other conductive structures and other insulative structures, the other stack structure comprising pillars vertically overlying the strings of memory cells, each pillar comprising an other channel material in electrical communication with the channel material of the strings of memory cells, and conductive contact structures vertically overlying the other stack structure, each conductive contact structure comprising an electrically conductive contact at least partially extending into the pillars and a portion extending outside of the pillars having a larger cross-sectional area than the pillars. Related microelectronic devices including self-aligned conductive contact structures, and related electronic systems and methods are also described.