Annexins are a family of calcium-dependent phospholipid-binding proteins. They are abundant in the eukaryotic kingdom. Though structurally well investigated in the last twenty years the in vivo function of the annexins is still unclear. The determination of the crystal structure of human annexin V was the first milestone in the structural investigation of this protein family. Succeedingly, a variety of three-dimensional structures of annexin crystals as well as of membrane bound annexins were solved. They should provide tools to understand the in vivo function of the annexin family. Based on the structural knowledge mutagenesis studies and biophysical investigations were started to elucidate possible functions of annexins like membrane binding and ion channel activity.