Ultrahigh-speed circuit applications of resonant tunneling diodes (RTDs) have been developed. One of the key concepts is the merged utilization of RTDs and high electron mobility transistors (HEMTs). The integration technology for InP-based RTDs and HEMTs has been developed. Another key technology developed is a circuit configuration using series-connected RTDs, driven by a clocked bias, in combination with HEMTs. Given this circuit concept, various kinds of edge-triggered flip-flop circuits and multiple-valued quantizers featuring high-speed operation and compact configuration have been constructed. By extending this circuit concept, an optoelectronic circuit using RTDs and a photodiode has also been developed. High-speed operations have been demonstrated, including a delayed flip-flop circuit operating at 35 Gbit/s, multiple-valued quantizers operating at 10 GHz, a 2-bit analog-to-digital converter operating at 5 GHz and an optoelectronic circuit that demultiplexes an 80 Gbit/s optical signal into a 40 Gbit/s electrical signal. The presented results clearly show the potentiality of RTD-based circuits for the construction of unprecedented ultrahigh-speed communications and signal processing circuits.