Proteins are an essential part of life and participate in virtually every process within cells. Proteins rarely act alone, and in recent years, the problem of reconstructing the proteins interactome from the large database of available proteins sequences has triggered much interest. Yet, this network has been shaped by various factors and constraints (chemical interactions between residues, evolution by mutations...). After an overview of the biological forces behind protein protein interactions, I will present a "first principle" statistical analysis, to infer putative interactions between proteins based on their sole amino-acids sequences.