Mountains and Clouds is a sculpture by Alexander Calder (1898-1976). It is the centerpiece of the towering, asymmetrical 90-foot atrium in the Hart Senate Office Building in Washington, D.C. The sculpture consists of painted aluminum and steel with a matte black finish.

During the construction of the Hart building in the 1970s, a panel of curators, charged with commissioning a sculpture for the atrium, selected Calder's sketch and model of Mountains and Clouds. Unfortunately, Calder died of a heart attack in 1976 after submitting his final model and designs. Mountains and Cloudswas completed from Calder's final designs and installed in the atrium in 1986, four years after the Hart building was first occupied.