In recent years, more and more compactifications have emerged whose existence depends crucially on the presence of internal sources to the supergravity fields, such as D-branes and orientifold planes. I will review some solutions of this type in various dimensions, and illustrate their applications to holography and potentially to the problem of finding de Sitter solutions.

I will review some recent progress about superconformal field theories in six dimensions. A simple class has an effective description in terms of a chain of gauge fields, coupled by tensors and hypermultiplets. Their holographic duals are now known analytically, and some precision checks can be performed, involving Weyl anomalies. An extension of this class can be studied using F-theory. All these theories have some features in common with the elusive theory describing M5-branes, and indeed suggest new phenomena, such as M5 fractionation.

Four-dimensional supersymmetric type II string theory vacua can be described elegantly in terms of pure spinors on the generalized tangent bundle T+T. This leads to constraints on M6 involving Hitchin's "generalized complex geometry". In this talk, we apply the same techniques to any ten-dimensional supersymmetric solution, not necessarily factorized as AdS4xM6 or Mink4xM6. I will describe a system of differential equations in terms of forms describing a "generalized ISpin(7) structure" is equivalent to unbroken supersymmetry, in both IIA and IIB. One of the equations in the system reproduces in one fell swoop all the pure spinors equations for four-dimensional vacua.

The Romans mass is a discrete parameter in type IIA string theory. It is perhaps the most mysterious piece of the theory: for example, its non-perturbative, M-theoretic interpretation is still not known. In this talk, we will review recent efforts to understand it through another non-perturbative tool: holography. We will see how this parameter modifies the recent holographic interpretation of certain string vacua via Chern-Simons theories. This leads to certain field theory results that, in turn, help find new families of supersymmetric vacua of string theory with negative cosmological constant.