The main reason to go beyond the 1-dimensional parton densities is to learn how quarks and gluons are distributed inside hadrons and how they move inside hadrons. The Generalized Parton Distributions (GPDs) and Transverse Momentum Dependent Parton Distributions (TMDs) only capture part of this. This has led to the natural extension to Generalized Transverse Momentum Distributions (GTMDs), which not only describe the spatial and momentum distributions, but also the correlations between them. In this talk I will discuss GTMDs and the proposed suggestions of how to access them experimentally. This will focus especially on the gluon GTMDs at small x and some of their phenomenology.