  In this chapter, the current status on baryon-baryon interactions such as
nuclear forces in lattice Quantum ChromoDynamics (QCD) is reviewed. In studies
of baryon-baryon interactions in lattice QCD, the most reliable method so far
is the potential method, proposed by the Hadrons to Atomic nuclei from Lattice
QCD (HAL QCD) collaboration, whose formulation, properties and extensions are
explained in detail. Using the HAL QCD potential method, potentials between
nucleons (proton and neutron, denoted by $N$) in the derivative expansion have
been extracted in various cases. The lattice QCD results shown in this chapter
include a Leading Order (LO) central potential in the parity-even $NN(^1S_0)$
channel, LO central and tensor potentials in the parity-even
$NN(^3S_1$-$^3D_1)$ channel, and a Next-to-Leading Order (NLO) spin-orbit
potential as well as LO potentials in the parity-odd channels. Preliminary
results at the almost physical pion and kaon masses, in addition to exploratory
studies on three-nucleon potentials, are presented. Interactions between
generic baryons including hyperons, made of one or more strange quarks as well
as up and down quarks, have also been investigated. Universal properties of
potentials between baryons become manifest in the flavor SU(3) symmetric limit,
where masses of three quarks, up, down and strange, are all equal. In
particular, it is observed that one bound state, traditionally called the
$H$-dibaryon, appears in the flavor singlet representation of SU(3). A fate of
the $H$ dibaryon is also discussed with flavor SU(3) breaking taken into
account at the almost physical point. Finally, various kinds of dibaryons,
bound or resonate states of two baryons, including charmed dibaryons, have been
predicted by lattice QCD simulations at the almost physical point.
