The application of measurement while drilling technology is fast becoming a crucial part of many successful horizontal drilling programs.The need for accurate directional and geological control is essential in order to meet predefined angle build rates and locate and stay within target horizons . This need is further emphasized in the smaller radius, slim hole, high angle wells that are now being drilled. This paper describes the application of a modular multi-sensor measurement while drilling system for use in most types of conventional and high angle directional wells for formation evaluation and directional control. ,Several case histories will be presented from diverse operating environments covering the following drilling applications: conventional directional build programs, medium radius operations, slim hole drilling, and other special drilling applications.Finally the paper will cover future measurement while drilling developments and the possible benefits for high angle and horizontal drilling techniques.