These notes are an introduction to implicit models of linear dynamical systems, with applications to modelling, control system design, and identification, intended for control-system engineers at the beginning graduate level. Because they are non-oriented, the models are particularly useful where causality is unknown or may change. They are implicit in all variables and closed under the algebraic operations, and hence are useful for computer-aided analysis and design. They possess the vector-matrix conceptual simplicity and computational feasibility of state-space equations, together with the generality of matrix-fraction descriptions, and admit of canonical forms for which the joint identification of system parameters and dynamic variables is linear. The notes simplify, generalize, and complement much recent work on "singular" or "descriptor" models, but do not duplicate it. Sections are included on realizations, canonical forms, minimal representations, algebraic design applications, quadratic optimization, identification, large-scale systems, and extensions to multi-dimensional and time-varying systems.