A microelectronic device comprises a stack structure comprising a vertically alternating sequence of conductive structures and insulative structures arranged in tiers, the stack structure divided into block structures separated from one another by slot structures, a staircase structure within the stack structure having steps comprising horizontal edges of the tiers, conductive contact structures in contact with the steps of the staircase structure, support pillar structures extending through the stack structure, and additional slot structures extending partially through the stack structure within one of the block structures, one of the additional slot structures extending between horizontally neighboring support pillar structures and closer to one of the horizontally neighboring support pillar structures than to an additional one of the horizontally neighboring support pillar structures. Related microelectronic devices, memory devices, and electronic systems are also described.