Buildings are expected to play a key role in the development and operation of future smart energy systems through real-time energy trade, energy demand flexibility, self-generation of electricity, and energy storage capabilities. Shifting the role of buildings from passive consumers to active players in the energy networks, however, may require closer cooperation between the energy and buildings sectors than there is today. Based on 23 semi-structured interviews and a web survey answered by key stakeholders, this study presents the views of the energy and buildings sectors on the current energy challenges in a comparative approach. Despite conflicting viewpoints on some of the issues, the energy and buildings sectors have similar perspectives on many of the current energy challenges. Reducing CO2 emissions is a shared concern between the energy and buildings sectors that can serve as a departure point for inter-sectoral cooperation for carbon-reducing developments, including the deployment of smart energy systems. The prominent energy challenges were identified to be related to low flexibilities in energy supply and use, which limit mutually beneficial cases, and hence cooperation, between the energy and buildings sectors today.