Software defined radios possess a necessary component: the RF frontend. The RF frontend, however, has the unfortunate reputation of being the source of most of the spurious responses seen in an SDR. Fortunately, managing these spurious responses can be explained with a few simple equations and examples. Direct conversion frontends presently dominate the SDR devices. As SDR applications move into higher frequencies and data converter sample rates increase, super-heterodyne architectures and low-IF architectures will begin to be more prevalent in SDRs. Each architecture has its pros and cons regarding spurious responses. This presentation introduces the theory behind these various RF frontend architectures with an emphasis on the sources of spurious responses. Knowing how spurious responses are generated can help determine the best means to minimize these effects.