Over the last decade biological research has changed completely. The reductionism approach of studying only a few biological entities at a time in the past is being replaced by the study of the biological system as a whole today. This requires that existing biological knowledge (data) is made readily available. Effective integration of biological knowledge from databases scattered around the internet and other information resources (for example experimental data) is recognized as a pre-requisite for many aspects of biological research. Systems for the integration of biological knowledge have to overcome several challenges: biological data sources may contain similar or overlapping coverage and the user of such systems is faced with the challenge of generating a consensus data set or selecting the "best" data source; different access methods to databases, different data formats, different naming conventions and erroneous or missing data. To address these challenges and enable effective integration of biological knowledge, the ONDEX system was created. ONDEX provides an integrated view across biological data sources to the user. Here the basic principles behind ONDEX are presented.