New business opportunities for mobile, wireless, and fixed networks are going to require managed packet based services; this requires SLAs that relate to the level of QoS purchased, and the measurement (monitoring) of information loss and delay at the packet level. In this chapter we investigate the two available measurement techniques: passive and active monitoring. We show that passive monitoring techniques can provide excellent accuracy with minimal computational overhead. However, it also has the disadvantage that it is necessary to have access to all the routers in all measured end to end paths, so severely limiting scalability. Alternatively active monitoring techniques can provide global reach; however it is critical that we go on to show that this technique has the disadvantage that (under many circumstances) the measured results may be very inaccurate. Finally we propose some ideas which may enhance accuracy.