  I explain the current status of parton-distribution-function (PDF) studies
and future experimental prospects on their determinations. First, unpolarized
PDFs of the nucleon are introduced as a field of precision QCD physics
including higher-order $\alpha_s$ corrections. Second, nuclear PDFs and
polarized nucleonic PDFs are discussed. Third, the determination of
fragmentation functions is explained. Forth, the three-dimensional (3D)
structure functions are discussed in connection with the origin of the nucleon
spin and gravitational form factors of hadrons. By the 3D structure functions,
gravitational sources and the origin of nucleon mass could be clarified in the
microscopic level of quarks and gluons. Furthermore, 3D structure studies of
hadrons could be used for clarifying internal structure of exotic hadron
candidates, nuclear composition of ultra-high-energy cosmic rays, and
color-entanglement phenomena. The PDF field will be developed further along
with progress in other fields of science.
