This article presents a theoretical advance research on the underground or illegal economy in the cash economy for the Colombian case, from the review of models of money demand (Williams Baumol), monetary offer (R.J. Barro) and the structure of a own model of inflation. To achieve this, several concepts were reviewed in the economic literature about: parallel or underground and illegal economy; the other economy; informal economy; hidden economy or in the shade; or informal economy, as several studies at national and international level that one or another way attempt to relate the underground economy with monetary demand for specific manner, regardless of the money supply and inflation, whose models (supply and inflation) are included in this article, for the first time in the Colombian case, the underground or illegal.