Reactions important for astrophysics, such as 7Be(p,γ)8B, 3He(α,γ)7Be or the famous 12C(α,γ)16O are hard or impossible to measure at energies at which they occur in the stellar environment. The measurements are typically performed at higher energies and then extrapolated to the energy of interest. Predictive first-principles nuclear theory of these reactions is then essential. Even if low energy measurements are achievable in underground laboratories such as LUNA, the beam-target experiments suffer from electron screening absent in stellar environment. Again, the predictive nuclear theory becomes indispensable to extract the correct physics. The ab initio approach Petr has been developing, No-Core Shell Model/Resonating Group Method (NCSM/RGM), reached the precision to address these issues. An example of recent (and first ever ab initio) investigations within the NCSM/RGM include the 3He(d,p)4He fusion calculations relevant to the primordial nucleosynthesis and the calculations of the 7Be(p,γ)8B capture reaction important for the solar neutrino physics and the Solar Model. Ab initio calculations of the 3He(α,γ)7Be reaction are ongoing.