Underground pedestrian networks in the city centers have been developed around the world, often because of the presence of an underground subway, but also because of the climate, like for the two largest Canadian cities, Montreal and Toronto.Some learnt and globally succeed to develop their underground space by a sustainable way because that space has been considered as valuable as streets and parks.Cities are the places to meet people, exchange ideas, do business or simply see the life goes on.It should be the same for their underground spaces.A successful underground city needs a large spectrum of activities down there, but also well-planned public spaces that have human scale, that are safe and lively.Experts spoke abundantly of the advantages and the inconveniences to use that space and even more on how to build in the underground.Unfortunately, few spoke about planning,regulation measures and design guidelines for this kind of development.This lecture will present, amongst other examples, the Montreal Underground city, which is one of the oldest and the most extensive indoor pedestrian network in the world.