Rotary Steerable directional drilling and logging assemblies deliver significant value to operators through improved drilling efficiency, precise well placement and effective formation evaluation. System failure is associated with a high cost and a high pain factor. Non-productive time (NPT) is used to re-run the failed bottom-hole-assembly (BHA); hole section objectives may be compromised and there is even the potential for loss of hole section, requiring a re-drill. This paper describes a risk-based approach to quantify and improve the reliability and performance of directional drilling bottom hole assemblies. Extensive field data gathered while drilling fifteen million feet of hole between 2003 to 2005 has been used to drive a reliability centered design and maintenance program with the objective of improving drilling performance, improving system reliability, increasing average rates of penetration and reducing NPT. All aspects of service delivery are considered, including mechanical and electrical system component performance, repair and maintenance procedures, job design and field operating practice. The approach has yielded measurable improvements in rotary steerable and logging while drilling bottom hole assembly reliability and performance. Industry metrics such as mean time between failure (MTBF), feet drilled per circulating hour (FPCH), feet drilled between failure (FBF), failure free runs and operating efficiency (ratio of NPT to operating time) are considered, benchmarking current performance and setting performance expectations for existing and future downhole systems. The industry has a need to standardize the method of evaluating downhole drilling system performance and to understand and quantify the benefits and risks associated with the application of a specific downhole system in a given downhole environment. This paper proposes such a standard approach, using novel BHA performance measures and an assessment of BHA complexity and well difficulty. Introduction The functionality and complexity of a Drilling and Measurement BHA has evolved at a fast pace. In the early nineties the standard motor/MWD BHA in general use at the time was often complemented by density neutron and resistivity logging while drilling tools. These BHA's were used to deviate the path of the wellbore, to acquire correlation logging while drilling data and to place the wellbore effectively with respect to geological and petrophysical horizons. Horizontal and highly deviated drilling allowed oilfields to be exploited with fewer wells. These techniques often caused the cost of each individual well to increase while total field development costs decreased significantly. The increase in sophistication of downhole drilling BHA's stimulated a focus in the industry on BHA reliability and several papers were published on the topic. [1,2]. One of the major outcomes of the focus was the definition and standard adoption of a common method of measuring BHA reliability - MTBF. In recent years introduction of rotary steerable systems and an array of sophisticated formation evaluation while drilling tools (formation pressure while drilling, nuclear magnetic resonance, azimuthal resistivity, azimuthal density, sonic) have again significantly increased the complexity of the BHA. The fast paced introduction of new technology supports the continuing trend of our customers to drill fewer, more efficient, productive and effective boreholes. At the same time drilling has moved into more hostile environments including deepwater, HTHP and areas with problematic, difficult formation types. Day rates for the fourth and fifth generations rigs required for these types of environments can rise to $350,000 per day and any amount of non-productive or hidden lost time causes significant pain to operators. BHA reliability and performance is therefore a significant factor in assessment of service supplier performance.