Intense sub-cycle optical pulse generation is one of the most sought-after advanced features of future laser developments, since not only does it symbol a breakthrough in ultrafast laser technology but it also opens up a gateway to a new regime of strong-field lightmatter interaction. As an important subject of strong-field physics studies nowadays, attosecond pulse technology will enjoy new opportunities brought by the emergence of intense sub-cycle optical pulses. Beyond directly generating extreme ultraviolet (XUV) isolated attosecond pulses, the tailored waveform of intense sub-cycle optical pulses can be employed for optimizing attosecond pulse generation. However, the more than one octave spanning spectrum of sub-cycle optical pulses makes the generation of such pulses extremely challenging. Coherent combination, or synthesis, of several few-cycle pulses of different colors is a promising technology for obtaining intense sub-cycle optical pulses. This article reviews the progress of studies on intense sub-cycle optical pulses, emphasizing the generation techniques based on optical waveform synthesis.