  Recent developments of theoretical studies on spin nanotubes are reviewed,
especially focusing on the $S=1/2$ three-leg spin tube. In contrast to the
three-leg spin ladder, the tube has a spin gap in case of the regular-triangle
unit cellwhen the rung interaction is sufficiently large. The effective theory
based on the Hubbard Hamiltonian indicates a quantum phase transition to the
gapless spin liquid due to the lattice distortion to an isosceles triangle.
This is also supported by the numerical diagonalization and the density matrix
renormalization group analyses. Furthermore, combining analytical and numerical
approaches, we reveal several novel magnetic-field-induced phenomena: N\'eel,
dimer, chiral and/or inhomogeneous orders, new mechanism for the magnetization
plateau formation,and others. The recently synthesized spin tube materials are
also briefly introduced.
