Résumé
In 1997, Z. Zhang and R.W. Yeung found the first example of a conditional information inequality in four variables that is not "Shannon-type". This linear inequality for entropies is called conditional (or constraint) since it holds only under condition that some linear equations are satisfied for the involved entropies. Later, the same authors and other researchers discovered several unconditional information inequalities that do not follow from Shannon's inequalities for entropy. In this paper we prove that some non Shannon-type conditional inequalities are "essentially" conditional, i.e., they cannot be extended to any unconditional inequality. We prove one new essentially conditional information inequality for Shannon's entropy and discuss conditional information inequalities for Kolmogorov complexity.