“Theorists say an intense enough laser field would rip photons into electron-positron pairs, dousing the beam. But no one knows whether it’s possible to reach that point.” The theory about electron-positron generation from vacuum is reviewed, also the developing of laser intensity. Schwinger predicted that electron-positron pair would be produced from the vacuum when the field near the critical QED field E s= m e2 c 3/ e ħ =1.32×1016 V/cm (the corresponding intensity I s=4.65×1029 W/cm2). The threshold to generate electron-positron pairs is about 5×1027 W/cm2 for single focused laser beam, and about 1026 W/cm2 for a pair of colliding laser pulses. When one electron-positron pair is generated, it would induce cascade effect under such strong laser field which would deplete energy of the field, so the highest intensity would be about 3×1026 W/cm2. The highest power people have reached is 5.13 PW, the corresponding intensity is about 1024 W/cm2, which was obtained by Chirped Pulse Amplification from Ti: Sapphire laser system. There are several projects in the world to generate 10–15 PW laser in the next few years, and several hundred PW to one Exawatt in the far future which would obtain laser intensity of 1026–1027 W/cm2. It is still not clear if the far future aim could be reached.