This paper is a survey of reachability and controllability results for disc
rete-time positive linear systems. It presents a variety of criteria in bot
h algebraic and digraph forms for recognising these fundamental system prop
el ties with direct implications not only in dynamic optimization problems
(such as those arising in inventory and production control, manpower planni
ng, scheduling and other areas of operations research) but also in studying
properties of reachable sets, in feedback control problems, and others. Th
e paper highlights the intrinsic combinatorial structure of reachable/contr
ollable positive linear systems and reveals the monomial components of such
systems. The system matrix decomposition into monomial components is demon
strated by solving some illustrative examples.