At all levels, biological complex systems have remarkable characteristics. In this paper, we describes some of these characteristics, such as the multi-level and nested structures of these complex systems, the multiple interactions between their constituents, their interaction functions which are often non-separable, their interaction networks which often are not graphs but hypergraphs, etc. We also emphasize the great difficulty - if not the impossibility - of defining a measure of complexity for these complex systems whose structure is itself dynamic.