  Heavy Quark Effective Theory splits a heavy quark momentum into a large fixed
momentum and a variable residual momentum, p = m_Q v + k. It thereby suffers a
redundancy of description corresponding to small changes in the choice of the
fixed velocity, v. The fact that full QCD is manifestly v-independent should
lead to a non-trivial constraint on the form of the effective theory, known as
Reparameterization Invariance. For spin-1/2 quarks, the precise form of the
constraint and its solution at the level of the effective lagrangian have
proven to be rather subtle, and the original proposal by Luke and Manohar has
been questioned. In this paper I employ a version of Heavy Quark Effective
Theory containing the ``anti-particle'' field as a non-propagating auxiliary
field, which greatly simplifies keeping track of v-dependence. This permits a
very simple derivation of Reparameterization Invariance from first principles.
The auxiliary field can also be integrated out to return to the standard
formulation of the effective theory, but with the effective lagrangian now
satisfying the full reparameterization constraint. I compare this result with
earlier proposals.
