Living systems are able to maintain internal order throughout life, a phenomenon known as homeostasis, by importing energy from the environment while exporting entropy. Homeostasis is achieved by the actions of cells that continually maintain their tissue environments and repair them when injured. Agent-based modeling demonstrates that homeostasis requires that the cells responsible for tissue maintenance receive feedback from the tissue concerning the results of their labors. It also demonstrates that a system capable of ongoing tissue maintenance automatically has the capacity to repair tissue injury, although temporary mechanisms that accelerate repair may be required by the demands of life. Agent-based models also provide insight into the conditions that predispose to the development of pulmonary fibrosis. Living systems can also be viewed as complex dynamic networks, raising the general question as to how such networks can fail to function normally. Three possibilities are identified—structural damage, a persistent external insult, and the existence of multiple stable network states (attractors). A Hopfield net model illustrates the conditions under which multiple stable states may exist when only one of these states corresponds to normality. Mixture theory allows tissue remodeling to be simulated over continuous time and space.