The first fruit of Marx's association with Engels was the heavily polemical The Holy Family, which was begun in the latter part of 1844, and was published towards the end of 1845. The bulk of the book is the work of Marx, and it documents Marx's final break with the rest of the Young Hegelians. It was followed shortly afterwards by The German Ideology, written in 1845–6, also primarily a critical work, but one in which Marx for the first time outlines a general statement of the tenets of historical materialism. From this time onwards, Marx's general outlook changed little, and the rest of his life was devoted to the theoretical exploration and the practical application of the views set out in this latter work.

The full text of The German Ideology was not published in the lifetime of Marx or Engels. In 1859, looking back to the period at which The German Ideology was written, Marx wrote that he and Engels were not disappointed that they could not get the work published: they ‘abandoned the work to the gnawing criticism of the mice all the more willingly’, since the main purpose – ‘self-clarification’ – had been achieved. Nonetheless, Marx explicitly refers to his ‘Critique’ of Hegel, and to the year 1844, as marking the most significant line of demarcation in his intellectual career.