Nineteenth-century German science was frequently involved in philosophical disputes and also in political issues. Most thinkers wanted their systems to be considered ‘scientific’, and Marx and Engels were no exceptions. However, they sharply distinguished their approach from that of the popularizing ‘materialist’ philosophers, Büchner, Vogt and Moleschott. In this paper we review the relation of Marx and Engels to these and other tendencies, both in ideas and in personal contacts, and show how they distinguished their ‘dialectical’ materialism from that which (...) they described as ‘vulgar’. (shrink)