  We show that quasiparticles in a magnetic field of arbitrary strength $H$ can
be described by field dependent parameters. We illustrate this approach in the
case of an Anderson impurity model and use the numerical renormalization group
(NRG) to calculate the renormalized parameters for the levels with spin
$\sigma$, $\tilde\epsilon_{\mathrm{d},\sigma}(H)$, resonance width
$\tilde\Delta(H)$ and the effective local quasiparticle interaction $\tilde
U(H)$. In the Kondo or strong correlation limit of the model the progressive
de-renormalization of the quasiparticles can be followed as the magnetic field
is increased. The low temperature behaviour, including the conductivity, in
arbitrary magnetic field can be calculated in terms of the field dependent
parameters using the renormalized perturbation expansion. Using the NRG the
field dependence of the spectral density on higher scales is also calculated.
