A major shift in energy systems has started due to drastic changes in climate and air quality, depleting resources, as well as rising social awareness to all these changes. As a result international protocols, such as the Paris agreement, are signed setting ambitious targets on both energy consumption and CO2 emissions. As industry is the responsible for a high share in energy consumption and environmental impact, such international treaties, combined with the regional and country based regulations put the industrial sector in the spotlight for energy and resource efficiency improvements. This thesis explores the issues that have been overlooked in the domain of industrial energy and resource efficiency. The first two chapters look at the energy consumption at the plant level, while the remaining chapters also consider interactions between plants, in the context of an industrial cluster. Chapter 1 presents the concept of heat exchange interfaces to assess the cost of heat integration within industrial plants at early design stages. Switching from one interface to another is linked to modifying existing heat exchangers and to the cost of additional heat exchange area requirement. An optimisation method is developed to consider the trade-off between the cost of switching interfaces and the operational benefits due to better heat integration. Chapter 2 takes the retrofit analysis of heat exchanger networks one step further, by introducing the plant layout in a mathematical formulation and considering further retrofit actions, such as moving heat exchangers, repiping streams, adding new heat exchangers and adding area to existing heat exchangers. Chapter 3 expands the boundaries of the retrofit problem to industrial clusters, considering interactions between the plants, such as sharing heat and resources. A method is proposed to simultaneously optimise the energy conversion technologies to be installed on the plants and the piping infrastructure for inter-plant exchanges. The method takes into account the locations of the plants and their impact, by considering heat losses, temperature and pressure drops, as well as the cost of piping, to prioritise recovery within and nearby the plants. Chapter 4 addresses the complexity of industrial retrofit investment planning with an optimisation method considering long time horizons. The method determines the commissioning and decommissioning time of the retrofit investments under given budget constraints. This thesis proposes a set of optimisation based methods to provide guidelines for the industries to reach short and long term energy and environmental targets. The results show that there is a large potential for energy and resource efficiency, protecting the economic interest of companies. Further potentials can be unlocked by sharing excess heat and resources with neighbouring industries and districts.