Silicon-based integrated circuit technology provides a great platform for enabling compact, efficient, low-power, chip-scale THz systems for new applications in sensing, imaging and communication beyond the niche scientific applications that the spectrum is currently known for. While this is partially facilitated by scaling that has pushed device cut-off frequencies (ft, fmax) up into the sub-THz and THz frequency range, the true paradigm shift in silicon integration is that it provides a unique opportunity to enable a new class of active THz electromagnetics realizable through a circuits-EM-systems co-design approach. At these frequencies, the chip dimension is several times larger than the THz wavelengths. This allows novel scattering and radiating surfaces that can be realized in a substrate that simultaneously supports a billion high-frequency transistors with the ability to generate, process and sense these signals. The ability to actively synthesize, manipulate and sense THz EM fields at deep subwavelength scales with circuits opens up a new design space for THz electronics. THz architectures emerging from this space are often multi-functional, reconfigurable and break many of the classical trade-offs in a partitioned design approach. This paper provides examples to illustrate this design methodology on THz spectrum sensing and synthesis and radiation of THz waveforms on-chip.