We discuss focus phenomena that may occur with seismic surveys. There are situations where all rays from a source are concentrated upon a single receiver location; this location is a point focus. There are also situations where all rays from a source are concentrated upon receiver locations distributed along a line; this situation is a line focus. In either case, the signal amplitude may be anamolously high at the focus and anamolously low at other places, making the data difficult to interpret.The point foci are subdivided into three types: the reflection points of the rays from the source to the unique receiver location can fall together; or they can be distributed along a line in the reflecting interface; or they can be distributed over the whole reflecting interface. The line foci are in two types: the reflection points of the rays from the source to any (fixed) receiver location on the line focus can fall together, or along a line in the reflecting interface.