Faced with countless threats that pose a danger to the continued existence of the human species, there has emerged a new field in philosophy known as Existential Risk Management. This discipline proposes to understand, quantify, classify and ultimately defeat the risks that exist and that threaten our continued existence. These philosophers accept that human life is good and that it should be promoted. And because of the tremendous value that is found in human life, they argue we should do whatever we can to avert our disappearance. Philosophical pessimists hold that life is always filled with suffering and that because of this to not exist is better than to exist. Yet once we are here, once we exist, it is in our interest to reduce the amount of suffering. So while pessimists sustain that nonexistence and the disappearance of the human species is the ultimate goal (insofar as it is the only way to defeat and terminate the suffering that conditions all of existence), this nonexistence is to be obtained by nonviolent means, voluntarily and only in full understanding of our existential predicament. In this dissertation I do two things. First, I define pessimism. This is essential because pessimism means different things to different people. Therefore, an important purpose of my work is to delimitate and establish an historically grounded definition that carves out the unique contributions that pessimism makes to philosophy. To do this, I survey the history of pessimism and lay out the main arguments made by pessimists. Two, I argue that philosophical pessimists can support the efforts made by the Existential Risk philosophers even though the reasons for doing so are different. And in so arguing I highlight the relevance and importance of the history of philosophy for contemporary debates.