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 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 field 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 which allows novel scattering and radiating properties in a substrate that simultaneously supports a billion high-frequency transistors that can generate, process and sense these signals. The ability to actively synthesize, manipulate and sense THz EM fields at 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 of a partitioned design approach. This paper provides examples to illustrate this design methodology on THz signal generation with beam-forming and spectrum control and THz spectrum sensing.