  We demonstrate that quantum instruments can provide a unified operational
foundation for quantum theory. Since these instruments directly correspond to
laboratory devices, this foundation provides an alternate, more experimentally
grounded, perspective from which to understand the elements of the traditional
approach. We first show that in principle all measurable probabilities and
correlations can be expressed entirely in terms of quantum instruments without
the need for conventional quantum states or observables. We then show how these
states and observables reappear as derived quantities by conditioning joint
detection probabilities on the first or last measurement in a sequence as a
preparation or a post-selection. Both predictive and retrodictive versions of
states and observables appear in this manner, as well as more exotic
bidirectional and interdictive states and observables that cannot be easily
expressed using the traditional approach. We also revisit the conceptual
meaning of the Heisenberg and Schr\"{o}dinger pictures of time evolution as
applied to the various derived quantities, illustrate how detector loss can be
included naturally, and discuss how the instrumental approach fully generalizes
the time-symmetric two-vector approach of Aharonov \emph{et al.} to any
realistic laboratory situation.
