  In these Lectures, I review various extensions of the Lambda-Cold Dark Matter
model, characterized by additional light degrees of freedom in the dark sector.
In order to reproduce the successful phenomenology of GR in the solar system,
these fields must effectively decouple from matter on solar system/laboratory
scales. This is achieved through screening mechanisms, which rely on the
interplay between self-interactions and coupling to matter to suppress
deviations from standard gravity. The manifestation of the new degrees of
freedom depends sensitively on their environment, which in turn leads to
striking experimental signatures.
