The marginal zone B-cell lymphoma, MALT-type (MZBL, MT) is a low-grade B-cell lymphoma which is predominantly localised in the stomach with a typical morphology and cytogenetic pattern. The coexistence of a diffuse large B-cell lymphoma (DLBCL) with an MZBL, MT in the gastrointestinal tract is defined as a composite lymphoma (ComL) and represents a fascinating model of lymphoma progression. In this review we focus on current aspects regarding the molecular characterisation of MZBL, MT and gastrointestinal DLBCL and their mutual relationships.