  We review the structure $D=6, N=1$ string vacua with emphasis on the
different connections due to T-dualities and S-dualities. The topics discussed
include: Anomaly cancellation; K3 and orbifold $D=6, N=1$ heterotic
compactifications; T-dualities between $E_8\times E_8$ and $Spin(32)/Z_2$
heterotic vacua; non-perturbative heterotic vacua and small instantons; N=2
Type-II/Heterotic duality in D=4 ; F-theory/heterotic duality in D=6; and
heterotic/heterotic duality in six and four dimensions.
