FDEMINV is a computer program that can be used to model and to interpret geophysical frequency-domain electromagnetic (FDEM) soundings using a horizontally layered earth model. The 1-D model is defined by the resistivity and thickness of a fixed number of layers and magnetic susceptibility can be assigned for one of the layers. The source is either a vertical or horizontal magnetic dipole. The receiver measures the vertical and horizontal components of the secondary magnetic field. The response can defined in different ways as a function of increasing frequency at fixed loop spacing (frequency sounding) or increasing loop spacing at fixed frequency (geometric sounding). Parameter optimization, which is based on linearized inversion method, can be utilized in 1-D interpretations.