Finding communities in networks is crucial to understand their structure and function, as well as to identify the role of the nodes and uncover hidden relationships between nodes. In this talk I will highlight the open problems that still keep the scientic community from having a shared set of reliable tools for the clustering analysis of real networks. In particular I will address the complexity of real community detection, due to the presence of overlapping communities and hierarchical structure, I will discuss the limits of null models at the basis of existing techniques and assess the delicate issue of testing the performance of methods.