Juan Domingo Sanchez Estop argues that Spinoza’s influence fundamentally shaped Althusser's philosophical project, providing key concepts and methods that Althusser used to radically rethink Marxism. The book traces five key 'detours and returns' between Althusser and Spinoza, showing how Spinoza’s anti-humanism, theory of reading, immanent causality, politics of the conjuncture, and rejection of determinism were mobilised at critical junctures in Althusser’s development. In the process, Estop uncovers a new 'Althusserian Spinoza', a thinker of practice and politics whose revolutionary potential remains to be explored.Bringing together published works, correspondences, and unpublished writings, this groundbreaking study sheds new light on Althusser’s theoretical trajectory and reveals the hidden Spinozist foundations of one of the 20th century’s most important Marxist thinkers.