This note describes the AT&T Labs PacketScope, a high performance sys tem for packet-level Internet measurement, which we have developed with the help of a great many others in AT&T Worldnetsm and AT&T Networking and Computer Systems. The NYC PacketScope, operational in June 1997, harvests about 10M packet headers per day on links between AT&T Worldnetsm and the external Internet. The Bridgeton, Missouri PacketScope, deployed in July 1997, can monitor a variety of traffic from the dial platform, web hosting and other servers, as well as gate way links to the external Internet. The data has a wide range of applications to AT&T Worldnetsm marketing, engineering, and service provisioning. One applic ation is described here: to the design and performance tuning of the IP over ATM infrastructure for AT&T's ATM common backbone, which will provide the network platform for AT&T WorldNet. Finally, we offer some remarks on ongoing simulation and analytic work.