This brief overviews the development of high-performance single-core voltage-controlled oscillators (VCOs) employing a multi-LC tank over the past two decades. From the circuit-design perspective, we study the key breakthroughs for constantly heightening the figure-of-merit and lowering the 1/f3 phase-noise corner, according to the noise source and transfer, narrow and wideband performance optimization, as well as one- and two-dimensional (1D/2D) tuning. We also aim to present the prospects of single-core VCOs.