Many needles in the haystack -- on the contribution of VLBI to cracking the FRB puzzle

It has been a little over fifteen years since the discovery of Fast Radio Bursts (FRBs) and we are still unsure about what type of object generates these enigmatic millisecond-duration bursts. The phenomenon is extremely common -- we expect several thousands of bursts per sky per day -- and we find roughly a dozen of new bursts each week. The bursts' properties tell us that they originate from cosmologic distances and that the emission must be coherent in nature. Broadly speaking, there are two types of FRBs: the repeating ones and those that apparently do not repeat. It is an open question whether the two populations share a common origin; some statistical studies indicate that this might not be the case. Amongst aspects such a the burst structure, bandwidth, duration and polarisation properties, a key ingredient required to solve the FRB puzzle is the local environment FRBs reside in. In combination with high-resolution optical data, VLBI-localisations of FRBs allow us to draw robust conclusions about the physical properties in the immediate surroundings of these sources and thus obtain clues about the possible progenitors. In this talk I will review our current understanding of FRBs with a particular focus on the contribution of VLBI in this endeavour