High-temperature superconductivity requires both strong electronic interactions, and a mechanism by which these interactions are prevented from inducing an alternative, static electronic order. Therefore the study of competing phases is an important part of the study of high-temperature superconductors. In underdoped cuprates, it is well-established that there are strong susceptibilities to both superconductivity and charge density wave order. It is not clear whether these orders are best described as mutually incompatible, different manifestations of the same pairing interaction, or different components of a composite order parameter. In this talk, I will present elastic and inelastic X-ray data on the underdoped cuprate YBa₂Cu₃O_6.67 (T_c = 65 K), that show compression along the a axis by ~1% induces static, three-dimensional CDW order. Upon reducing the temperature, the CDW disappears and is replaced by superconductivity, indicating strong competition between these states.