  In \cite{Pe1}, \cite{Pe2}, \cite{AP1}, \cite{AP2}, and \cite{AP3} sharp
estimates for $f(A)-f(B)$ were obtained for self-adjoint operators $A$ and $B$
and for various classes of functions $f$ on the real line $\R$. In this note we
extend those results to the case of functions of normal operators. We show that
if $f$ belongs to the H\"older class $\L_\a(\R^2)$, $0<\a<1$, of functions of
two variables, and $N_1$ and $N_2$ are normal operators, then
$\|f(N_1)-f(N_2)\|\le\const\|f\|_{\L_\a}\|N_1-N_2\|^\a$. We obtain a more
general result for functions in the space $\L_\o(\R^2)=\big\{f:
|f(\z_1)-f(\z_2)|\le\const\o(|\z_1-\z_2|)\big\}$ for an arbitrary modulus of
continuity $\o$. We prove that if $f$ belongs to the Besov class
$B_{\be1}^1(\R^2)$, then it is operator Lipschitz, i.e.,
$\|f(N_1)-f(N_2)\|\le\const\|f\|_{B_{\be1}^1}\|N_1-N_2\|$. We also study
properties of $f(N_1)-f(N_2)$ in the case when $f\in\L_\a(\R^2)$ and $N_1-N_2$
belongs to the Schatten-von Neuman class $\bS_p$.
