Abstract: Given a domain , consider a Brownian motion starting from an interior point , the harmonic measure describes where the Brownian motion exits from the domain. Information of harmonic functions is also encoded in : given a continuous function , let be its harmonic extension in , we have . A natural question arises: how is related to the surface measure of the boundary? In other words, does the Brownian traveller see things differently from our naked eyes? The answer to this question in turn reveals geometric information of the domain. This talk will review some cornerstone results in this topic for the plane and higher dimensions.