With the emergence of Web service mash-ups (Web applications that integrate different data sources), online data integration and aggregation is increasingly becoming the online norm for both commercial and non-commercial users. With such widespread adoption of data integration from discrete sources, the question emerges as to whether the resultant mash-up can be considered as trustworthy. This paper explores the concepts behind Web service mash-ups to determine the factors influencing their trustworthiness. The focus is on examining data quality and data assurance issues for both the data providers and the mash-up consumers.