Hybrid halide perovskite solar cells (PSCs) giving over 22 % power conversion efficiencies (PCEs) have attracted considerable attention. Although perovskite plays a significant role in the operation of PSCs, the fundamental theories associated with perovskites have not been resolved in spite of the increase in research. In this Minireview, we assess the current understanding, based on the first-principles calculations, of structural and electronic properties, defects, ionic diffusion, and shift current for CH NH PbI perovskite, and the effect of ionic transport on the hysteresis of current-voltage curves in PSCs. The shift current connected to the possible presence of ferroelectricity is also discussed. The current state-of-the-art and some open questions regarding PSCs are also highlighted, and the benefits, challenges, and potentials of perovskite for use in PSCs are stressed.