We present a review of heterotic-type I string duality. In particular, we
discuss the effective field theory of six- and four-dimensional
compactifications with N>1 supersymmetries. We then describe various duality
tests by comparing gauge couplings, N=2 prepotentials, as well as
higher-derivative F-terms. Based on invited lectures delivered at: 33rd Karpacz
Winter School of Theoretical Physics ``Duality, Strings and Fields,''
Przesieka, Poland, 13 - 22 February 1997; Trieste Conference on Duality
Symmetries in String Theory, Trieste, Italy, 1 - 4 April 1997; Cargese Summer
School ``Strings, Branes and Dualities,'' Cargese, France, 26 May - 14 June
1997.Comment: 14 pages, LaTeX, espcrc2.st