A method of forming a microelectronic device comprises forming a microelectronic device structure. The microelectronic device structure comprises a stack structure comprising insulative structures and additional insulative structures vertically alternating with the insulative structures, a dielectric structure vertically extending partially through the stack structure, and a dielectric material vertically overlying and horizontally extending across the stack structure and the dielectric structure. Portions of at least the dielectric material and the dielectric structure are removed to form a trench vertically overlying and at least partially horizontally overlapping a remaining portion of the dielectric structure. The trench is substantially filled with additional dielectric material. Microelectronic devices, memory devices, and electronic systems are also described.