Since the introduction of the parton distribution function by Feynman few decades ago, much progress has been made in understanding the 3D structure of hadrons. How are quarks and gluons distributed inside hadrons? Are these distributions process- dependent? What is their contribution to proton spin? In order to answer these questions, one needs to properly consider hadronic matrix elements with both longitudinal and transverse dependence, which present very subtle issues. In my talk I will review these issues, explain how they are resolved, and discuss previously mentioned questions and others.