A significant amount of mass is lost from the Earth's atmosphere through ions escaping from the polar ionosphere. Due to spacecraft charging effects, in situ measurements using traditional plasma instruments are typically not able to detect the low energy part of the outflow. Recent advances in instrumentation and methodology, combined with comprehensive data sets from the Cluster constellation of spacecraft have provided far better opportunities to assess the role of the low energy ions. With this new technique, it is possible to bypass detection problems caused by spacecraft charging effects, and provide quantitative, in situ estimates of cold ion density and outflow velocity. In this chapter, we give an overview of these advances and highlight some of the key results based on this methodology. The results corroborate earlier findings that polar rain and the open polar cap is the primary source of cold outflow, but we also find enhanced cold outflow from the cusp and auroral zone though, in particular during disturbed geomagnetic conditions. The transport of cold ions is mainly governed by the convection, and most of the outflowing ions are transported to the nightside plasma sheet and recirculated in the magnetosphere. Transport times are of the order of two to four hours from the ionosphere to the nightside magnetospheric plasma sheet. Direct loss along open field lines downtail into the solar wind only takes place during quiet magnetospheric conditions with low or stagnant convection. Only about 10% of the total cold outflow is directly lost downtail into the solar wind.