Principles of physics allow us to calculate synthetic data from earth models. Such calculations are said to solve ``forward'' problems. In real life we are generally interested in the reverse calculation, i.e., computing earth models from data. This reverse calculation is called ``inversion.'' The word ``inversion'' is derived from ``matrix inversion.'' Despite its association with the well-known and well-defined mathematical task of matrix inversion, echo sounding inversion is not simple and is often ill defined. Inversion promises to give us an earth model from our data despite the likelihood that our data is inaccurate and incomplete. This promise goes too far. Inversion applied to perfect data, however, can give a perfect result, which makes inversion more appealing academically than processing by adjoint modeling.