It has been close to 60 years since the first operational silicon solar cell was demonstrated. However, the last 15 years have seen large improvements in the technology, with the best confirmed cell efficiency improved by over 50 percent. The main drivers have been improved electrical and optical design of the cells. Improvements in the first area include improved passivation of contact and surface regions of the cells and a reduction in the volume of heavily doped material within the cell. Optically, reduced reflection and improved trapping of the light within the cell have had a large impact. Features have increased silicon cell efficiency to a recently confirmed value of 24.7 percent. Over recent years, good progress has been made in transferring some of the corresponding design improvements into commercial product with commercial cells of 17-18 percent efficiency now commercially available, record values of a mere 15 years ago. The theory supporting these improvements in bulk cell efficiency shows that thin layers of silicon, only a micron or so in thickness, should be capable of comparably high efficiency.