  Many one--dimensional quantum systems, in particular interacting electron and
spin systems, can be described a Luttinger liquids. Here, some basic ideas of
this picture of one--dimensional systems are briefly reviewed. I then discuss
the effect of interchain coupling for a finite number of parallel chains. In
the case of spin chains coupled by exchange interactions, the low--energy
properties are radically different according to whether the number of coupled
chains is even or odd: even number of chains have a gap in the spin
excitations, whereas odd numbers of chains are gapless. The effect of
interchain tunneling is analyzed for two and three coupled chains of itinerant
fermions: for repulsive interactions, the two--chain system is ``universally''
found to be a d--wave superconductor, with a gap in the spin excitation
spectrum. On the other hand, for three chains the ground state depends both on
the boundary conditions in the transverse direction and on the strength of the
interactions. Weak repulsive interactions in all cases lead to dominant
superconducting pairing of d--type. An example of a three--leg spin ladder with
a spin gap is proposed. A general scheme to keep track of fermion
anticommutation in the bosonization technique is developed.
