  The differential decay spectrum $d\Gamma/dE_e dq^2$ for the semileptonic
decay of an unpolarized hadron containing a b-quark, and the decay spectrum
$d\Gamma/dE_e dq^2 d\!\cos\theta$ for polarized $\Lambda_b$ decay are computed
to second order in the $1/m_b$ expansion. Most of the $1/m_b^2$ corrections
have a simple physical interpretation, which is discussed in detail. The
implications of the results for the determination of $V_{ub}$ are discussed.
The decay spectra for semileptonic decay of hadrons containing a c-quark are
also given. There is a subtlety in the use of the equations of motion at order
$1/m_b^2$ in the heavy quark expansion which is explained.
