The article focuses on challenges involved with system integration that are specific to digital libraries and discusses the available solutions that meet the digital library requirements and the digital library prototypes. System integration includes pre-integration, identification of schema matching, schema integration and source-data integration subprocesses. The first task in the pre-integration process is to identify the different data models and extract their schema. The next task is to select a data model as the common model. The final task is to transform every data model of the underlying sources into the common model. The schema integration process starts with identifying the portion of the source schema to be included in the integrated schema and then defining mappings between the integrated schema and the corresponding portion of the source schema. Specific issues to integration in digital libraries are a direct result of their characteristics. These include, massive amount of structured, semi-structured and unstructured data, multimedia nature of data, frequent modification of information sources and need to cater to different user characteristics.