Progressive politics is facing a crisis that has been several decades in the making. The 'crisis of social democracy' in the wake of the 1980s neoliberal revolution has been followed shortly afterwards by the demise of the liberal 'end of history' since the wars and financial crises of the 2000s. Rapid increases in the size, complexity, and fluidity of modern societies have disrupted old communities and identities, and brought new ones into being to challenge them. Cultural tensions have risen alongside older economic and political divisions to the forefront of ideological contests. The result has been a very specific form of polarisation: the resurgence of the far right, and a fragmentation of the left towards the centre and the extremes. To confront this crisis, the progressive left must reevaluate its approach to ideological and strategic competition. The last major precedent it has for what can happen when the left is fragmented and the right has turned towards extremism stems from one of the darkest periods of world history. The rise of fascism, the schism of the left between social democracy and communism, and the nadir of a disoriented liberalism eventually allowed bloodshed and destruction to take place on a scale never seen before. In this light, the contemporary left must work to overcome its divisions and bring about ideological unity across its various manifestations-socialist and liberal, green and anarchist, republican, regionalist, anti-racist, feminist, or pro-LGBTQ*. The left needs clear visions and proposals for how to combine its forces and face the tasks ahead with strength and determination. This book makes the case for today's progressives to adopt a policy of 'left unity' across parties and all other parts of the left movement, and outlines strategies for how the contemporary left can start to build a 'progressive alliance'. These strategies are inspired by the spirit of past efforts to achieve progressive unity, but they are motivated by the needs and possibilities of the crisis the left faces today. It is for progressives of all colours to learn from them what they can before it is too late.