 He first contributed to fundamental advances in time-frequency analysis, with the development of comprehensive approaches aimed at analyzing, decomposing, and processing nonstationary signals  .He also took an active part in the development of wavelet theory since its very beginning, with highly cited seminal contributions to the multiresolution analysis of scaling processes   that paved the way to numerous applications in domains as diverse as biomedical engineering or internet traffic modeling.More recently, while revisiting a number of fundamental issues in nonstationary time series analysis by new, data-driven approaches , he moved to the study of specific complex systems involving human activities, with a shift towards network-based approaches.