Observations suggest the Universe underwent two periods of accelerated expansion, one at very early times and one today. I will discuss the observations and focus on the recent period of accelerated expansion, discussing a potential explanation in terms of modified gravity and in particular the chameleon mechanism. I will show how a screening mechanism results in gravity being modified in sparse environments, like the cosmos,  but not in the solar system. The implications will be discussed for cosmology, astrophysics and in the laboratory. Experiments searching for such modified gravity theories discussed.