Lead-based halide perovskites are most prominent candidates for emerging opto-electronic applications. In this talk I will overview ‘in silico’ efforts towards finding new Pb-free semiconductors that are alternatives to traditional halide perovskites, for which ab initio methods successfully revealed a series of new compounds within the so-called halide double perovskites family and vacancy ordered perovskites. Among these, I will discuss the case of Cs2AgBiBr6 which exhibits the narrower indirect band gap of 1.9 eV, and Cs2AgInCl6, the only direct band gap semiconductor, yet with a large gap of 3.3 eV. All of them exhibit low carrier effective masses and consequently, are prominent candidates for a range of opto-electronic applications such as photovoltaics, light-emitting devices, sensors, and photo-catalysts. We will specifically outline the computational ab initio design strategy that led to the synthesis of these compounds, and particularly focus on the insights we can get from first-principles calculations in order to facilitate the synthesis, improve their opto-electronic properties and the in-silico identification of compounds with properties that are similar to the lead-halide perovskites. The newly developed concept of analogs will lead us to identify a new oxide double perovskite semiconductor: Ba2AgIO6, which exhibits an electronic band structure remarkably similar to that of our recently discovered halide double perovskite Cs2AgInCl6, but with a band gap in the visible range at 1.9 eV. The developed approach will be employed to describe the opto-electronic properties of challenging complex materials like the case of Ag-Bi halide double salts, develop a consistent symmetry-based approach to model these, and employ the model to establish their potential performance as solar-cell absorbers. Finally, I will further address the exploration of the phase space of vacancy-ordered double perovskite like Cs2SnI6, Cs2TeI6 and also present the case of Zr-based compounds as stable alternates to Cs2TiX6 with X=Br,I, which exhibit lighter charge carrier effective masses. I will show state-of-the-art calculations to accurately describe their optical, excitonic properties and its fine-structure, in order to unveil the limitations and opportunities for their potential technological application.