Geomechanical analysis is a mechanism for understanding the complex interactions that occur in a deforming system, leading to the ability to predict how that deformation impacts the key physical properties of the rocks, such as those related to fluid flow. Specifically, geomechanics enables us to determine how assemblages of rocks will respond to a loading arrangement, provided that we can also stipulate the complete suite of mechanical behaviors for all of the components of the system. The roles of geomechanical processes, in terms of how they influence the fluid system, are crucial elements in many applied subject areas and especially so in the consideration of seals. In fact, the interaction between geomechanical processes and pore fluids is bidirectional, via effective stress, and through permeability, which is itself primarily controlled by geomechanical processes that alter the pore network during deformation. By acknowledging this bidirectional interaction, we can consider how both rock deformation and fluid flow represent the transfer of energy through complex natural systems. The nonreversable coupling between these two processes leads to highly nonlinear system responses, such as the formation and operation of seals. Poroplasticity, which integrates the data and concepts derived from decades of rock mechanics testing, is a material description that provides the critical link to allow us to make realistic geomechanical predictions about seals. This coupled geomechanical + fluids approach, based on poroplasticity, is applied here to explain the formation and predict the capacity of top seals. The approach is also used to show how our understanding of fault seals can be improved by considering the evolution of deformation-altered materials both during and after faulting. In both situations (top seals and fault seals), the creation (or failure) of seals is primarily related to alterations of the pore system by mechanical processes (which may assist in setting the stage for a chemical or diagenetic overprint). Throughout, geomechanical processes play a first-order role in governing seal behavior.