In this chapter, I will discuss open worlds. One extreme situation that impossible worlds represent is one that is not closed under any non-trivial i-principle. We call them open worlds. In contrast to inconsistent worlds, the idea of open worlds is motivated by the thought that no L-M principle is general enough to close impossible worlds. Thus, impossible worlds are worlds that are not closed under any L-M entailment. Although defining impossibility as openness is too liberal, thinking about impossible worlds as open worlds is a natural route to take, even when one is modal realist.