The increased demand for high throughput lithography with Front To Back-side Alignment (FTBA) capability has led ASML to develop the FTBA functionality within its current platform. This option is named 3DAlign and aims at an FTBA overlay of 500 nm. To relate a position on the back side of a substrate to its front side, two back-side marks are projected from the back to the front side by two optical branches. In this way, the images of the back-side marks have the proper orientation and magnification to be aligned by the standard ASML alignment hardware. The metrology challenge is to calculate the back-side mark positions in front-side (exposure) coordinates and to compensate for the systematic errors introduced by the optical branches. An absolute relation between the front- and back-side positions on the substrate is obtained by calibrating the system with a special 3DAlign calibration wafer. This is a wafer that has marks on both sides with a known offset. In this paper, the basic ideas and algorithms for the FTBA metrology are discussed.