Brain diseases such as cancers, neurodegenerative disorders, or trauma are frequently diagnosed with imaging modalities and sometimes with intracranial biopsies. Treatment response is similarly monitored, along with clinical indications. While these technologies provide important windows into the disease state, they fail to provide us a detailed molecular portrait of the disease and of the changes taking place during therapy. Exosomes are virus-sized nanovesicles derived from the endosomal system and are released extracellularly from essentially all cell types. Exosomes contain intracellular entities (proteins, nucleic acids, metabolites), membrane proteins and lipids, and even extracellular proteins bound to them. Exosomes may be considered as mini-surrogates of their cells of origin, with some content common to all cells/exosomes, but some of the content would be cell-specific. These vesicles are found in all biofluids in humans, and are thus accessible to "liquid biopsy" with harvest of vesicles from such fluids. Current challenges are to identify disease-related markers or panels of markers to distinguish the disease state. Here we will show examples of brain tumor markers found in/on exosomes from cell culture and patient sera, and we will suggest that aspects of the biology of disease may have a relevant place in the search for biomarkers.