A method of manufacturing a lateral semiconductor device comprising a semiconductor body (2) having top and bottom major surfaces (2a, 2b), the body including a drain drift region (6a) of a first conductivity type. The method includes the steps of forming a vertical access trench (20) in the semiconductor body which extends from its top major surface (2a) and has a bottom and sidewalls; forming at least one horizontal trench (16) extending within the drain drift region (6a) which extends from a sidewall of the vertical trench (20) in the finished device; and forming a RESURF inducing structure (22) extending within the at least one horizontal trench. In this way, vertically separated lateral RESURF inducing structures are formed without encountering problems associated with known techniques for forming RESURF structures.