Web services provide programmatic interaction between organisations within large heterogeneous distributed systems. Using recent experiences of constructing and enhancing a data integration system for the health domain, based on web services, we draw conclusions about new problems for program comprehension. These derive from the fundamentally dynamic and distributed nature of the environment. We suggest several key research topics for program comprehension, arguing that these are crucial if software constructed from web services is to be supportable over a long period. Finally, we briefly summarise some wider conclusions about understanding web services at the application domain level.