The liberalisation process of the electricity sector of most European countries was carried out in stages, starting with the customers with the highest consumption and highest voltage levels.

In Portugal, a similar methodology was followed and the market opening took place gradually between 1995 and 2006. Since 4 September 2006, all consumers in mainland Portugal can choose their electricity supplier.

The supply of electricity to the consumers, both qualitatively and quantitatively, is the fundamental objective of the activities developed by the National Electricity System (SEN) under the principles of rational and efficient use of resources in all the electricity sector activities, from the production of electricity to its supply to the final consumer.

The electricity supply is subject to public service obligations, which are the responsibility of all who participate in the electric sector, and includes:

• safety, regularity and quality of the supply
• guarantee that the service provided is universal
• guarantee the connection of all clients to the networks
• protection of consumers namely in terms of tariffs and prices;

In addition, it is assured to all the stakeholders in involved the several activities of the electricity sector:

• freedom of access to the exercising of the activities;
• no discrimination;
• equal treatment and opportunities; 
• impartiality of decisions;
• transparency and objectivity in rules and decisions; 
• access to information and safeguarding of the confidentiality of commercial information considered to be sensitive
• freedom to choose their electricity supplier.

The main activities of SEN can be described as follows:

• Generation: electricity is produced using various technologies and different primary sources of energy (coal, gas, fuel oil, diesel, water, wind and biomass, among others). In Portugal Mainland, electricity is produced by several distinct companies. 
• Transmission: after generation, electricity must be transported at Very High Voltage level from the power plants to the supply points. In Portugal mainland, transmission is handled by a single company – Rede Eléctrica Nacional (REN).
• Distribution: in addition, electricity is distributed through High, Medium and Low Voltage levels through distribution networks where most of the end consumers are connected. In Portugal mainland most of the distribution networks are handled by one single company, EDP Distribuição, and also by some Low Voltage electricity distribution operators. 
• Supply: electricity suppliers are responsible for managing the relations with the end customers, including billing and energy-related advice and support. In Portugal, mainland electricity can be sold through suppliers on the liberalised market and under specific circumstances on the regulated market through the supplier of last resort.

The following activities of the electric sector are subject to regulation:

• Transmission;
• Distribution;
• Supplier of Last Resort;
• Logistic Operations for Switching Supplier and
• Management of Organised Markets.