We see them in our everyday lives. They make skies and sunsets even more beautiful, inspiring painters all over the world. But what are clouds? What are the physical processes occuring within a cloud? Do they all look alike, or are there different types of clouds? Why? In this course, we will first review well-understood aspects of the spatial distribution of clouds on our planet, and some fundamentals of cloud physics. We will then discuss key open questions about clouds in our current and in a warming climate. We will notably review the effect of clouds on climate, how this effect is quantified, as well as recent theoretical advances on the response of clouds to climate change.