Present InP bipolar transistors attain 1.1 THz fmax; InP field-effect transistors attain 1.5 THz. These can support emerging applications in 100-300 GHz wireless communications and imaging radar, 400-1000 Gb/s wireline and optical communications, and high-frequency instruments. After summarizing the applications and the required circuit and transistor performance, I will review transistor design, present transistor performance, and the design of next-generation THz bipolar and field-effect transistors.