Data and considerations are presented about the sustainability of astronomical observatories in the Spanish territory. While still not comparable to other science institutions, the energy consumption of large telescopic installations is high. To partially compensate it, effort has been put into producing in situ green energy, taking advantage of the fact that observatories are located in sites with strong solar irradiation. Sustainably, however, is transversal and pivotal to any activity, and all its pillars (environmental, economic, and social) should be addressed in order to build an integral strategical plan for sustainability. This is what we have done at the Gran Telescopio Canarias, in collaboration with the University of La Laguna. It includes diverse areas of action such as energy, mobility, water, biodiversity, IT/OT, procurements, consumption, equality, safety, health and welfare, and alliances with other institutions and companies. The development of a strong sustainability culture, however, is perhaps the most important step to promote the dramatic change that is needed to stop the over-exploitation of our planet.