Exosomes are extracellular vesicles first described as such 30 years ago and 
since implicated in cell-cell communication and the transmission of disease 
states, and explored as a means of drug discovery. Yet fundamental questions 
about their biology remain unanswered. Here I explore what exosomes are, 
highlight the difficulties in studying them and explain the current definition 
and some of the outstanding issues in exosome biology.