In the last six years the power conversion efficiency of solar cells made with metal halide perovskite semiconductors has soared from less than 7% to over 23%. Only four other families of semiconductors have ever reached this efficiency. Perovskites are very attractive compared to those families because they can be deposited from solution at low cost, function well even when they contain a high density of defects, and have the right bandgaps to be used in tandem solar cells. In tandems one semiconductor harvests the photons in the solar spectrum that have higher energy and generates a large voltage while another semiconductor harvests the photons with lower energy. Using this strategy makes it possible to improve the efficiency by approximately 50%. In this seminar I will discuss several strategies we have used to tune the band gap, an unusual reversible light-induced phase separation we discovered, the numerous implications of having of mobile halogen vacancies, our progress in making tandem solar cells with > 25 % efficiency and several strategies for making the solar cells stable. A pathway to stable low-cost 30 % efficient solar cells will be presented.