The topic of system security, and in particular that of cybersecurity differs in a critical way from the other concerns we have about systems. Though concerns like safety and resil- ience do have challenges associated with design, realization, and validation to an ever changing operating environment, security faces an ever evolving adversary. When faced with constantly changing conditions under which a system must continue to deliver its function, designers attempt to model those conditions and test their design against that model. Modeling also becomes important from a measurement standpoint. In order to assess systems and determine their overall risk, their overall security posture, design coun- termeasures, and then reassess systems to determine the effectiveness of countermeasures in a provable, reproducible, repeatable quantitative manner, we must be able to model the security, vulnerability, and risk of these systems. In this chapter we will introduce new modes of modeling for security adversaries and discuss some basic foundations for adversary modeling. We also discuss how connectivity to other systems, their interactions increase complexity. These complexities also need to be identified and modeled to understand the derivative effect on the overall security posture.