  A code ${\cal C}$ is $\Z_2\Z_4$-additive if the set of coordinates can be
partitioned into two subsets $X$ and $Y$ such that the punctured code of ${\cal
C}$ by deleting the coordinates outside $X$ (respectively, $Y$) is a binary
linear code (respectively, a quaternary linear code). In this paper
$\Z_2\Z_4$-additive codes are studied. Their corresponding binary images, via
the Gray map, are $\Z_2\Z_4$-linear codes, which seem to be a very
distinguished class of binary group codes.
  As for binary and quaternary linear codes, for these codes the fundamental
parameters are found and standard forms for generator and parity check matrices
are given. For this, the appropriate inner product is deduced and the concept
of duality for $\Z_2\Z_4$-additive codes is defined. Moreover, the parameters
of the dual codes are computed. Finally, some conditions for self-duality of
$\Z_2\Z_4$-additive codes are given.
