A method of forming a microelectronic device comprises forming line structures comprising conductive material and insulative material overlying the conductive material, the line structures separated from one another by trenches. An isolation material is formed on surfaces of the line structures inside and outside of the trenches, the isolation material only partially filling the trenches to form air gaps interposed between the line structures. Openings are formed to extend through the isolation material and expose portions of the insulative material of the line structures. The exposed portions of the insulative material of the line structures are removed to form extended openings extending to the conductive material of the line structures. Conductive contact structures are formed within the extended openings. Conductive pad structures are formed on the conductive contact structures. Additional methods, microelectronic devices, memory devices, and electronic systems are also described.