Directional drilling (sometimes known as slant drilling outside the oil industry) is a method used in oil drilling for exploration and extraction of crude oil, where the direction of the drill string is forced out of an essential vertical direction. In modern petroleum engineering, especially in off-shore operations, directional drilling has become the norm rather than the exception, and the path of the drill can be controlled with great precision, and be extended horizontally only limited by the total length of the drill string.

Since slant drilling is very difficult to trace for outsiders of the operation, the technique has frequently been used to steal oil resources from neighbouring oil fields. Several cases are known, the one with the most spectacular consequences being slant drilling performed by Kuwait. This was one of the reasons given by Iraq for invading that country in 1991 (see Gulf War).