Key experiences that led to the development of Karl Marx's theories include his interactions with philosophical, economic, and political thinkers during his education in Germany and France. His exposure to the industrial revolution and the rise of capitalist societies deeply influenced his understanding of social classes and material conditions. Furthermore, Marx's work as a journalist allowed him to analyze the impact of capitalism on workers and society. Additionally, his collaboration with Friedrich Engels was crucial in shaping his ideas on communism and class struggle, culminating in seminal texts like The Communist Manifesto.