Amino acids are among the numerous cues governing host plant acceptance by aphids, through immediate or delayed behavioral and nutritional effects. Proteins are encountered by aphids along the whole process of stylet penetration and ingestion, but their action on aphid behavior and physiology is practically unknown. The functions of these 2 classes of biochemicals on aphid feeding are reviewed, with special emphasis on the molecular–cellular level of the aphid–plant interaction and their consequences on analytical and behavioral methodologies.