  We first show that every operation possesses an unique dual operation and
measures an unique effect. If $a$ and $b$ are effects and $J$ is an operation
that measures $a$, we define the sequential product of $a$ then $b$ relative to
$J$. Properties of the sequential product are derived and are illustrated in
terms of L\"uders and Holevo operations. We next extend this work to the theory
of instruments and observables. We also define the concept of an instrument
(observable) conditioned by another instrument (observable). Identity,
state-constant and repeatable instruments are considered. Sequential products
of finite observables relative to L\"uders and Holevo instruments are studied.
