Extracellular vesicles, particularly small exosomes with a size between 40 and 120 nm, contain complex information. Most importantly, exosomes may transfer this information to recipient cells and alter their cellular programme. If decoded, this information reveals ongoing malignant processes within the organism such as drug resistance or metastases formation (functional biomarker). As this information is composed of RNA, DNA fragments, proteins and metabolites, these molecules may also serve as molecular biomarker. Exosomes are generated from parental cells, frequently under disease conditions. Thus, exosomes share significant information with their origin including mutations of the malignant lesion. Tumour characteristics can therefore be retrieved in circulating exosomes, which opens a complete new approach in cancer diagnosis: the liquid biopsy.