An understanding of the formation is essential for effective drilling, casing and completion of oil and gas wells. Several methods and many tools are available to obtain information about the formation. Some of these tools collect or provide downhole data during the drilling process and are, therefore, termed measurement-while-drilling (MWD) tools. This paper describes a new MWD tool which records formation data during the drilling process. process. System hardware is described and its operation and rig impact outlined. A discussion of the theoretical implications of measuring formation data during the drilling process is presented. Field data are presented and compared with wireline data. From these data, excellent MWD log quality is demonstrated. MWD gamma ray logs are shown to have a superior bed resolution to those produced by wireline, for most cases, and the resistivity sensor, contained in the recording system, is demonstrated to be the most versatile MWD resistivity tool commercially available. It is seen from field history that recording formation data while drilling is a cost-effective and reliable method of obtaining formation information when compared to other available methods. Introduction A variety of MWD tools is commercially available which provide services ranging from only directional through multi-sensor packages which include various combinations of hole angle and direction, formation gamma and resistivity, and downhole temperature and pressure and weight and torque on bit. Except for the recording system discussed in this article, all commercial MWD systems depend on some form of mud-pulse telemetry for communication of the data from downhole to the surface. There are many cases when real time transfer of data is required. Tool face measurement during steering and WOB, for instance, would be of little use if not provided in real time. However, information obtained during drilling can provided in real time. However, information obtained during drilling can be of value when available at the completion of each bit run. Using this approach, a complex and costly portion of the MWD system can be eliminated, with concomitant advantages in cost, reliability and rig impact. Many applications do exist for recording formation characteristics (lithology) during the drilling process. A recording MWD service can be used when drilling surface hole to identify shallow hydrocarbons and eliminate a wireline log. High-angle and difficult holes are easily logged with a recording MWD tool when wireline logging is difficult, costly or impossible.