  This handbook provides a comprehensive review of
transverse-momentum-dependent parton distribution functions and fragmentation
functions, commonly referred to as transverse momentum distributions (TMDs).
TMDs describe the distribution of partons inside the proton and other hadrons
with respect to both their longitudinal and transverse momenta. They provide
unique insight into the internal momentum and spin structure of hadrons, and
are a key ingredient in the description of many collider physics cross
sections. Understanding TMDs requires a combination of theoretical techniques
from quantum field theory, nonperturbative calculations using lattice QCD, and
phenomenological analysis of experimental data. The handbook covers a wide
range of topics, from theoretical foundations to experimental analyses, as well
as recent developments and future directions. It is intended to provide an
essential reference for researchers and graduate students interested in
understanding the structure of hadrons and the dynamics of partons in high
energy collisions.
