  This paper describes the design goals and engineering efforts that led to the
realization of AMBER (Astronomical Multi BEam combineR) and to the achievement
of its present performance.
  On the basis of the general instrumental concept, AMBER was decomposed into
modules whose functions and detailed characteristics are given. Emphasis is put
on the spatial filtering system, a key element of the instrument. We
established a budget for transmission and contrast degradation through the
different modules, and made the detailed optical design. The latter confirmed
the overall performance of the instrument and defined the exact implementation
of the AMBER optics.
  The performance was assessed with laboratory measurements and commissionings
at the VLTI, in terms of spectral coverage and resolution, instrumental
contrast higher than 0.80, minimum magnitude of 11 in K, absolute visibility
accuracy of 1%, and differential phase stability of 1E-3 rad over one minute.
