Unlike double-jobbing, clandestine employment is often the only job that many workers have. It is carried out on the fringes of the law or outside it altogether, and takes three main forms - the undeclared employment of workers, undeclared self-employment, and undeclared multiple jobholding.
Clandestine employment particularly involves the unemployed, migrant workers, pensioners, unregistered self-employed workers, housewives, houseworkers, temporary staff, students and children. Their numbers are hard to estimate.
Undeclared income was estimated to be 13 percent of Belgium's GDP in 1999. This was almost three times the share of neighbouring Netherlands, though lower than Italy and Greece.