This thesis presents the analysis and design of monolithically integrated mixer circuits intended for the application in 77 GHz automotive radar systems in Silicon- Germanium technology. A new and advanced Silicon-Germanium process technology was accessable that provides high speed bipolar transistors. This cost-efficient bipolar technology opens up new fields of microwave applications. The frontends of today's automotive radar sensors are manufactured using either discrete mircowave circuits or chipsets based on compound semiconductors. If a low cost Silicon- Germanium solution is found, automotive radar systems can be manufactured very cost-efficiently. Thus saftey and comfort systems will enter not only the premium car segment but will be employed in every car, similar to the airbag or safety belt. However, by now only a few circuits have been investigated that operate in this frequency domain. In this thesis design concepts for mixers based on Silicon- Germanium at millimeter-wave frequencies are thoroughly investigated. This study includes the analysis and design of a direct conversion mixer with high linearity at 77 GHz. Additionally, the feasibility of high level integration at 77 GHz using Silicon-Germanium is shown. After investigating different building blocks, an integrated receiver frontend for millimeter-wave applications is designed, manufactured and measured. This circuit is among the first that shows the potential of high level integeration in Silicon-Germanium in this frequency regime.