Drilling for oil and gas has evolved significantly the past decade. Wells are now drilled in deep water, and some wells reach more than 10 km from production platforms. The equipment is loaded towards its design limits, and, resulting in a high economic risk.The book resolves a number of technical issues that are important to understand for successful drilling of critical wells. Subjects such as buoyancy and effects of wellhead pressures, well friction as drag and torque, and effects of weIlbore deviation are presented in detail. Because sticking of drill pipe is a costly drilling problem, one chapter derive equations to analyse these problems, and also defines three different methods to free a stuck drillstring.Mechanistic analyses of pipe stresses and failure is presented, and the book also contains chapters on depth measurement errors, downhole pumps and motors and advanced weIl control.The book is a must for anyone working in modern well technology.