Event-Triggered Control (ETC) is a control method where the controller is only updated when necessary. The control inputs are kept fixed until a state-dependent event triggers their re-computation. The triggering condition is designed to guarantee the stability and desired performance of the control system. This prevents the excessive use of scarce communication and energy resources. To fully take advantage of these savings when dealing with a (physically) distributed system, it is vital to decentralise the triggering condition. A decentralised triggering condition can be checked locally at the nodes of the system using the locally available states, eliminating the need to send updated sensor measurements to a central location constantly. However, the decentralisation of the triggering condition needs to be optimised for the state of the system to fully take advantage of the possible resource savings. This thesis provides a framework to optimally decentralise quadratic triggering conditions for Periodic Event-Triggered Control (PETC) systems in the absence and presence of bounded disturbances. In addition, a method of constructing a region-based map of such optimisations is provided, making it feasible to implement the optimisation approach on larger systems without needing more powerful hardware.