Engels played a crucial role in shaping Marx's famous work, Capital, by applying materialist dialectics to analyze the relationship between capital and wage labor. Engels continued this research after Marx's death, contributing to the enduring academic value of Capital. His key contributions include correspondence with Marx, annotations, and critiques of the bourgeoisie's response to the book. Capital remains a powerful tool in critiquing capitalist society, with Engels helping to develop and defend Marx's theories.