Photosystem I, extensively studied in the past decade, was shown to be homologous in all photosynthetic organisms of the higher plants type. Its core complex was found to be highly conserved through evolution from cyanobacteria to higher plants. The genes coding for the subunits of CCI were isolated and the resulting sequences provided information about secondary structural elements. These suggested secondary structures enabled the prediction of the topology of these subunits in the photosynthetic membrane. Structural studies using both electron microscopy and X-ray crystallography, on isolated particles as well as on the complexes in the photosynthetic membrane, led to a better understanding of the overall structure of CCI. Recently two forms of three dimensional crystals of CCI were obtained. These crystals contain all the original components of CCI (both protein and pigments); these components have not been altered by crystallization. It is expected that a detailed crystallographic analysis of these crystals, together with biochemical, spectroscopical and molecular biology studies, will eventually lead to the elucidation of the high resolution structure of the photosystem I core complex and to the understanding of the exact role and mode of action of this complex in the photosynthetic membrane.