  As in two and four dimensions, supersymmetric conformal field theories in
three dimensions can have exactly marginal operators. These are illustrated in
a number of examples with N=4 and N=2 supersymmetry. The N=2 theory of three
chiral multiplets X,Y,Z and superpotential W=XYZ has an exactly marginal
operator; N=2 U(1) with one electron, which is mirror to this theory, has one
also. Many N=4 fixed points with superpotentials W \sim Phi Q_i \tilde Q^i have
exactly marginal deformations consisting of a combination of Phi^2 and (Q_i
\tilde Q^i)^2. However, N=4 U(1) with one electron does not; in fact the
operator Phi^2 is marginally irrelevant. The situation in non-abelian theories
is similar. The relation of the marginal operators to brane rotations is
briefly discussed; this is particularly simple for self-dual examples where the
precise form of the marginal operator may be guessed using mirror symmetry.
