Proportionality in defense is a relation between the good and bad effects of a defensive act. Stated crudely, proportionality requires that the bad effects of such an act not be excessive in relation to the good. If this seems simple, the apparent simplicity is an illusion. This chapter explores some of the hitherto unappreciated complexities in the idea of proportionality. It explains how a requirement of proportionality differs from a requirement of necessity, distinguishes among various types of proportionality, and examines the ways in which proportionality in defense differs from proportionality in punishment. The chapter also suggests that certain good or bad effects may have less weight than others, or even no weight at all, in the assessment of proportionality. Finally, the chapter argues that proportionality is not just a matter of the consequences of action, but is also sensitive to the ways in which consequences are brought about.