Data aspects in workflow systems did not yet receive the same attention as
process aspects. Various kinds of data are processed in workflow system:
from case data to process data,  from internal data to access to external
databases or document exchanges in inter-organizational workflows. We propose
a uniform treatment of all kinds of data  in workflow definition and provide
abstraction mechanism which allow transparent access to all kinds of data in
a uniform way. We use XML as data access language in our workflow definition
language WDL-X. The concept contributes to transparency of data location and
logical and physical data independence of workflow systems. It facilitates
the reuse of predefined activities and subworkflows on different data sets
and eases the interaction of a workflow with its environment by abstracting
from the actual representation of data.