The current state-of-the-art and the various design tradeoffs encompassing the variety of low-noise microwave and millimeter-wave receiver "building blocks" which have evolved during the past two decades are described. Key examples of these are the high-idler non-cryogenic parametric amplifier, the gallium arsenide field-effect transistor (GaAs FET) amplifier, and the image-enhanced Schottky-diode mixer. It is then shown how this inventory of building blocks can best be integrated into optimum receiver configurations for application in a multiplicity of future and present microwave and miltimeter-wave communications, RADAR, and radiometer systems.