  There are an infinite number of type IIB superstrings in ten dimensions,
called (p,q) strings, that are labeled by two distinct string charges. They can
form string junctions and string networks. These are the key to understanding
the Coulomb branch BPS spectrum of N=4 super Yang--Mills theories in four
dimensions. Nongravitational theories that are analogous to type IIB
superstring theory in some respects are the ADE (2,0) six-dimensional theories
on the Coulomb branch. Each of them contains a finite number of half-BPS
superstrings, which can be labeled by the root vectors of the Lie algebra. When
the rank is greater than one, these strings can also form string junctions and
string networks. The relationship between N=4 super Yang--Mills theories in
four dimensions and the (2,0) theories in six dimensions can be explored by
utilizing the duality between type IIB superstring theory compactified on a
circle and M-theory compactified on a torus.
