  This contribution is intended as a `rough guide' to the Hipparcos Catalogue
for the non-expert user. Some general aspects of the use of astrometric data
are discussed as well as Hipparcos-specific applications. We discuss when and
at what level one may expect systematic errors to occur in the Hipparcos
Catalogue. Next we discuss the question of the interpretation of the measured
parallaxes in terms of distances and luminosities of stars. What are the biases
one should be aware of and how can these be corrected? When using the
astrometric data to study the statistics of stars one should take the full
covariance matrix of the errors on the astrometric parameters into account. We
explain how to do this and discuss the specific case of a moving cluster.
Finally, we address the question of the correlation of astrometric parameters
over a given region of the sky. At present the Hipparcos Catalogue contains no
identified systematic errors.
