Palestinian Islamic Jihad (PIJ) is one of the most important yet least understood Palestinian armed factions, both in terms of its history and ideology. Labelled a terrorist organization by the US and the EU, it has grown to become the second largest armed movement in the Gaza Strip and the third largest in the Occupied Palestinian Territories. Using a wealth of primary sources, this book traces the history of PIJ from its origins in the early 1980s to today. By looking at how the group was established, how it has developed in theory and practice, and how it understands religion and politics, Skare seeks to answer the key question of why the PIJ still exist despite the presence of its more powerful sister movement Hamas. In doing so, he fills an important empirical gap in the literature on Palestinian Islamism.