This chapter outlines the principles upon which strategies for fungal and bacterial resistance may be based, to evaluate the types of genetic manipulations which may lead to increased resistance. Some fungal pathogens have acquired virulence by being able to detoxify the phytoalexins the host produces as a part of its defensive arsenal. A basis of information exists for engineering modified phytoalexin structures which may be resistant to detoxification, or for transferring a phytoalexin biosynthetic pathway from one plant to another which lacks that particular pathway. Transgenic plants expressing either novel proteins from foreign organisms or overexpressing a part of their own defensive arsenal have been engineered, tested in both laboratory and field situations, and evaluated for disease resistance. Pathogenesis-related proteins are low-molecular weight proteins which accumulate to significant levels in infected plant tissues. Plants, and indeed other organisms, may contain antimicrobial proteins not necessarily associated with induced defense responses, which are potential subjects for engineered protection strategies.