The growing complexities of the current energy price crisis and environmental problems are leading to an acceleration in reductions in energy consumption. Stimulating the adoption of energy efficiency is one of the strategies formulated by the international community to reduce energy consumption and greenhouse gas emissions. Buildings in the EU are responsible for 40% of our energy consumption and 36% of greenhouse gas emissions. Improving energy efficiency in buildings therefore plays a key role in attaining the ambitious goal of carbon-neutrality by 2050. Huge investments in energy efficiency are required to achieve energy savings and climate goals. However, despite its significant monetary benefits and environmental advantages, levels of EE in buildings are generally low. This is the so-called energy efficiency gap. Many reasons exist for it, which can be mainly grouped into market, behavioural and other failures. And different energy efficiency policy instruments can be used to address those failures. If energy efficiency leads to significant reductions in energy consumption (and bills), why do residential and non-residential buildings invest so little in it? How should policy makers encourage investments in energy efficiency? What effective ways are there of making energy efficiency policies effective and accepted by all stakeholders? By answering these overarching research questions, the dissertation¿s main goal is to study the effects of energy efficiency policies and to understand how these policy instruments can be designed to promote effective, cheaper reductions in emissions and energy consumption in households and hotels, mainly in the context of Spain. To that end, this dissertation integrates and combines different methodologies, i.e. semi-quantitative approaches through the use of focus groups and surveys to understand behavioural complexity; and a quantitative econometric approach based on hedonic price method to provide evidence of the effectiveness of EE labels. We find that the application of policy packages may be useful for less coercive policy instruments (especially for households) and for ambitious EE targets. Specifically, ambitious technical standards and specific regulation would ensure that energy is saved. Environmental education and information policies seem to be useful in helping consumers to make better decisions. Additionally, in the light of variation in policy acceptability for economic instruments, energy tax could be combined with subsidies or other revenue recycling schemes. Findings suggest that various policy instruments can be used to help achieve EE targets, but good policy design and excellent implementation are needed, considering behavioural complexity on the part of key stakeholders and features of the policy instruments