As a PhD student I started working in the Local Theory of Banach Spaces, which is the part of Functional Analysis that studies the geometry of finite-dimensional subspaces - the
so-called local structure - of Banach spaces to gain information on the geometry of the space itself, that is, its global structure. Of special interest to me is the local structure of the
classical Banach spaces Lp, in particular the finite-dimensional subspaces with a symmetric or unconditional basis. Despite using functional analytic tools, combinatorial and probabilistic
methods are an integral part.
The Local Theory of Banach Spaces is naturally connected to Asymptotic Convex Geometry, which studies the geometry of convex bodies as the dimension tends to infinity.