The nature of trophic shifting between predation and phytophagy is discussed. The evolutionary pattern of predation in Heteroptera is discussed in light of several kinds of adaptations that relate to predatory or phytophagous specialization. These specializations include special behaviors, morphological, biochemical and physiological adaptations, as well as special features such as symbiotes, salivary glues and sheath materials, and development of venoms. Specialization includes the number, shape and direction of stylet dentition, the teeth pointing backward in predators, but cutting into the food source in some phytophages. Biochemical adaptations may be specific for predation, including venoms and phospholipase A₂, or they may be adaptive for both predation and specialized niches in phytophagy such as proteinases and lipases. Plant feeding specializations include true amylases and pectinases. When specialized enzymes for plant digestion (e.g., amylase) are found in carnivorous insects, it is strongly suggested that plant feeding is of more than casual, facultative importance. Symbiotes have been demonstrated as essential in phytophages that specialize in relatively nutrient-poor plant compartments such as xylem or phloem. However, symbiotes are also found in hematophagous insects, but their relationship with their hosts is unknown. A great deal of attention has been paid to stylet sheath-flange-glue materials, with previous supposition that these secretions are strictly related to plant feeding, mainly in Pentatomomorpha. However, many, if not all, pentatomomorph predators (e.g., lygaeids and pentatomids) produce stylet flanges, and some predatory members of Cimicomorpha produce stylet flanges. The need for understanding trophic switching is discussed from the perspective of economic pest control.