The consecutive steps of tumor growth, local invasion, intravasation, extravasation and invasion of anatomically distant sites are obligatorily perpetrated through specific interactions of the tumor cells with their microenvironment. Lumican, a class II small leucine-rich proteoglycans (SLRP) has been designated key roles both in extracellular matrix (ECM) organisation and as an important modulator of biological functions. This review will critically discuss lumicans' roles in tumor development and progression. We will especially focus on correlating lumicans' expression and distribution in tumor tissues with: (1) the organization of the tumor matrices; (2) tumor cell signaling and functions; (3) tumor cell-matrix interface; (4) tumor angiogenesis; and (5) lumicans potential roles in tumor-associated inflammatory response. Present knowledge of lumicans' biology provides a fundamental platform upon which to build and deepen our understanding of lumican function in tumorigenesis in order to be able to design credible anti-tumor approaches.