In designing and evaluating human-machine systems, cognitive models can be used to (a) provide design principles and (b) guide the construction of experiments. In this paper, we present an information processing model of cognition that we have used extensively in designing and evaluating interfaces and autonomy modes. This model uses a conventional description of short-term memory, but treats long-term memory as a collection of mental models specific for particular tasks. Working memory includes components of both short term and long term memory; short term memory acts as a "scratch" pad for an activated subset of long-term memory. We review this model and discuss how it has been used in several human-robot systems.