The paper deals with a new and very promising method of 2.5-D high-frequency modelling of seismic wavefields. The method consists in tracing 3-D rays through a 2-D structure (not varying in one direction). The 2.5-D approach is more efficient then 3-D modelling giving exactly the same results with no additional approximation. Basic theory is briefly described with the emphasis on certain differences and simplifications with respect to the fully general 3-D computations. The method is illustrated on one simple numerical example. A lot of possible applications are suggested.