Unilateral conduct laws differ across jurisdictions, but all prohibit a firm possessing dominance/substantial market power from engaging in exclusionary conduct. This prohibition does not protect consumers by directly addressing the exercise of market power, but rather protects consumers indirectly by addressing conduct that maintains or strengthens the position of dominance/substantial market power. Enforcing this prohibition presents the two fundamental questions addressed by this Chapter: What is dominance/substantial market power? And what makes conduct exclusionary? A jurisdiction’s unilateral conduct law, court decisions, and policy statements could have controlling force on how a competition agency answers these two questions, but this Chapter addresses them from first principles. Principles set out here can be of value despite the constraints of statutes, precedents, and guidance documents. This Chapter benefitted from teleconferences with, and from submissions from, ICN members and NGAs on these two fundamental questions.