Surface gas measurements to identify gas bearing zones have been in use since the 1920s. Early well-site sampling methods were inaccurate and inconsistent, but had little effect on results since measurement and analysis methods were equally poor. The introduction of gas chromatography in the 1950s created the demand for gas samples more representative of the gas content of the drilled formation. New techniques were developed, but even these methods of continuous gas extraction at the well site were recognized to be inconsistent. However, the basic gas extraction system developed then is still in use in the field today. The paper presents a critical evaluation of on-line gas monitoring systems as currently used in the drilling industry. A more efficient real-time monitoring system that combines state-of-the-art technology with established sampling elements has been developed. Special emphasis was put on the adaptability to oilfield operations. Qualitative and quantitative interpretation of the monitoring results are presented and field experience with the new system is evaluated. Recent case studies are critically discussed with an approach to formation evaluation and the correlation between well logging and gas logging. The presented gas monitoring system provides good performance under field conditions and yields representative results for the definition of the bottomhole gas signature. The results of the gas logging operation - if applied correctly - prove to be a valuable basis for volumetric gas analysis and subsequent well testing, thus time and cost reduction is achieved. Mobile and rugged technology, fast rig up and low maintenance make the system very flexible and customer friendly.