By continuity, inequality (1) extends to any function belonging to the homogeneous Sobolev space H 1 (R d ). It is well known that the constant (d − 2) 2 /4 in (1) is sharp but not attained. The literature concerning various versions of Hardy's inequalities and their applications is extensive and we are not able to cover it in this short paper. We only mention the classical paper of M. Sh. Birman [1], the article of E. B. Davies [4], and the book of V. Maz ya [11]. Among many applications of inequality (1) we would like to mention that, in combination with the Schwarz inequality, it impliesThis estimate takes a particularly symmetric form if in the second integral on the lefthand side we use the Parseval formula for the Fourier transform p u of the function u:This inequality expresses the Heisenberg uncertaintly principle, which states that a nontrivial L 2 -function and its Fourier transform cannot be simultaneously very small near the origin. Hardy's inequalities were also studied for some subelliptic operators (see, e.g., [6,7,2,3,5,12,9]), in particular, for the sub-Laplacian on the Heisenberg group H. This group is the prime example in noncommutative harmonic analysis, and we refer to [13] for the background material.