Lead halide perovskite (LHP) has gained tremendous attention from both academia and industry due to its excellent optical and electrical properties. To date, the power conversion efficiency (PCE) of a small area LHP solar cell has reached 25.5%. However, lead (Pb2+) element in perovskite compounds is toxic and harmful to the environment and human health. Additionally, LHP exhibits a larger energy bandgap (E-g) that is not within the optimal range in the Shockley-Queisser (S-Q) maximum efficiency limit, which has further limited the maximum attainability of the device performance. In order to reduce Pb2+ concentration in perovskite compounds and alter E-g close to the optimal range S-Q efficiency limit, tin (Sn2+) has been proposed to fully or partially substitute Pb2+ due to their similar ionic radii and outer electronic configurations. Even though the power conversion efficiencies (PCEs) of Sn2+ and mixed Pb2+/Sn2+ perovskite solar cells (PSCs) have seen rapid improvement in recent years, they are still far behind that of Pb2+ PSCs because the high number of defects in Sn2+ based halide perovskites induces poor morphology and poor optoelectronic properties in the perovskite films. Therefore, in this review, we will provide the challenging issues of Sn2+ halide perovskites and methodologies (i.e. device engineering) to boost the efficiency and stability of Sn2+ and mixed Pb2+/Sn2+ based single-junction PSCs. At the same time, we also show future perspectives and remaining challenges in this field, with an aim to help further progression of efficient and stable Sn2+ and mixed Pb2+/Sn2+ PSCs.