  We study surfaces constructed from groups of units in quaternion orders
$\Lambda$ over the integers in real quadratic fields k. A short presentation of
some general theory of such surfaces is given, in particular, we construct
certain ``modular curves'' on the surfaces and examine their properties. We
apply our results to some surfaces related to the case $k=Q(\sqrt{13})$ and
$d(\Lambda)=(3)$, and find their place in the Kodaira classification of
algebraic surfaces.
