A microelectronic device includes a stack structure comprising a vertically alternating sequence of insulative structures and conductive structures arranged in tiers. At least one slit region divides the stack structure into blocks. Each block comprises an array of active pillars. Along the at least one slit region is a horizontally alternating sequence of slit structure segments and support pillar structures. The slit structure segments and the support pillar structures each extend vertically through the stack structure. Additional microelectronic devices are also disclosed as are related methods and electronic systems.