Open data is receiving considerable attention because of its potential for public and private sector innovation. Various governments have the policy of providing data sets to the public via open data portals. Data sets are published in a format defined by a source, which makes it difficult to discover a useful data set and requires user interpretation of structure and semantics. Meta data insertion and semantic annotation address these problems, but are not yet widely implemented for open data. Also privacy issues and commercial sensitivity have to be addressed, leading to (technical) interventions like access restrictions and billing functions.