Lead-free halide double perovskites with the formula of A₂B⁺B³⁺X₆ are received significantly growing interest due to their potential to address the toxicity and instability issue of lead-based perovskites APbX₃. They show high stability, a wide range of possible combinations, rich substitutional chemistry, and attractive optoelectronic properties, making them successfully applied in various optoelectronic devices, such as solar cells, photodetectors, light-emitting diodes, etc. However, their device performance still lags far behind that of the state-of-the-art lead-based perovskite ones. Thus, we first summarize the main fundamental challenges that limit high-efficiency optoelectronic from both the materials development and device fabrication, including large bandgap, indirect bandgap nature, parity-forbidden transitions, low electronic dimensionality, weak or no photoluminescence, short carrier diffusion length, difficulties to fabricate and dope/alloy HDP films, contact issues, high defect density in thin films, etc.¹ Equally important, we highlight current possible solutions to these challenges, such as metal doping/alloying, B-site metal ions order to disorder, B-site metal ions arrangement, post-treatment, thermal evaporation, appropriate charge transport layers, etc.¹ Following this thinking and taking the large bandgap issue as an example, we successfully modify the bandgap and/or optical absorption properties of several double perovskites through metal doping, B-site metal ions order to disorder, and thermal heating. Specifically, we tune the bandgap of Cs₂AgInCl₆ over a range from 2.8 eV to 1.6 eV by Fe³⁺ alloying.² For Cs₂AgBiBr₆, its optical absorption edge is broadened from 610 to around 860 nm by incorporating Cu in the lattice, which is attributed to the sub-bandgap formation rather than bandgap narrowing.³ Besides metal doping/alloying, we also develop a crystallization control approach to narrow the bandgap of Cs₂AgBiBr₆ by simply increasing the crystal growth temperature from 60 ^oC to 150 ^oC. As a result, its bandgap reduced from 1.98 eV to 1.72 eV, which is the lowest reported bandgap for Cs₂AgBiBr₆ at ambient conditions. The underlying reason is hypothesized to be related to the increased level of Ag–Bi disorder.⁴ We also observe a remarkable and fully reversible thermochromism in the double perovskite Cs₂NaFeCl₆, indicating its bandgap can be simply adjusted by temperature. The bandgap of Cs₂NaFeCl₆ crystal is reduced from 2.5 to 1.87 eV when the temperature increases from 6.8 to 423 K, significantly exceeding the values reported in lead-based perovskites and many conventional group-IV and III-V semiconductors. The first-principles calculations reveal that this dramatic thermochromism is attributed to a strong electron-phonon coupling rather than thermal expansion or Na/Fe order-disorder transition. Until now, halide double perovskites are still in their infancy and require enormous efforts to realize their full potential.