The current European energy regulation, aligned with the European Union energy strategy and targets for the next decade, requires large industrial companies to regularly assess their energy performance and implement energy efficiency improvement measures. In many cases, these energy reviews fulfill minimum criteria for energy audits set by the legislation, and focus on the optimisation of the energy conversion units and utilities distribution. Opportunities for energy savings within production processes are missed, which can also lead to an inadequate hot and cold supply system. Existing methods for energy reviews in the petrochemical sector do not feature the holistic and systematic aspects required to deeply analyse and improve industrial sites down to the production units level. The lack of time and human resources, combined to the availability and reliability of data, are additional barriers preventing detailed studies to take place. This thesis presents a comprehensive methodology to carry out detailed energy review of petro- chemical plants, in accordance with energy management and auditing standards requirements. This methodology comprises three main steps: the energy consumption analysis, the targeting of the heat recovery potential and the identification and evaluation of energy saving opportunities to reach this target. A top-down approach is undertaken in the first step, with the objective of translating the raw energy consumption of the system into process units heating and cooling demand. In doing so the mass and energy flows are mapped and the efficiency of the entire energy chain is characterised in a structured way. The focus on the process requirements allows to understand how much, where and why energy is consumed. In this first step, guidelines and heuristic rules are defined to reduce the required time for data collection. A data consistency check in the form of key mass and energy balances ensures the validity of data and a good control of the energy flows of the system. In the second step, a novel methodology for the definition of the minimum approach temperature in pinch analysis is presented. By considering the characteristics of each process hot and cold stream individually, together with the economic parameters of the system, the heat recovery potential is refined and the minimum energy consumption targets are closer to what can be achieved economically. From the results of the pinch analysis, the objective of the third step is to reach the energy targets through the identification of energy saving opportunities. A bottom up approach is defined to look for options starting from the process operating parameters and heat integration towards the optimisation of the energy conversion and distribution system. Waste heat recovery through heat pumping being a recurring identified opportunity, a heat transformer system is proposed, coupling a mechanical vapour compression cycle to an organic Rankine cycle. Integrated to the polyethylene slurry production, this system allows to recover the residual reaction heat and produce steam without importing electricity from the grid. In doing so the energy consumption is reduced by 50%. The proposed methodology was developed, tested and refined on around 10 different petrochemical sites, enabling a comprehensive analysis of their energy performance and leading to the identification of promising energy saving opportunities to increase the energy efficiency of their production.