High-energy astrophysics is a relatively young scientific field, made possible by space-borne telescopes. During the half-century history of x-ray astronomy, the sensitivity of focusing x-ray telescopes - through finer angular resolution and increased effective area - has improved by a factor of a 100 million. This technological advance has enabled numerous exciting discoveries and increasingly detailed study of the high-energy universe - including accreting (stellarmass and super-massive) black holes, accreting and isolated neutron stars, pulsar-wind nebulae, shocked plasma in supernova remnants, and hot thermal plasma in clusters of galaxies. As the largest structures in the universe, galaxy clusters constitute a unique laboratory for measuring the gravitational effects of dark matter and of dark energy. Here, we review the history of high-resolution x-ray telescopes and highlight some of the scientific results enabled by these telescopes. Next, we describe the planned next-generation x-ray-astronomy facility - the International X-ray Observatory (IXO). We conclude with an overview of a concept for the next next-generation facility - Generation X. The scientific objectives of such a mission will require very large areas (about 10000 m2) of highly-nested lightweight grazing-incidence mirrors with exceptional (about 0.1-arcsecond) angular resolution. Achieving this angular resolution with lightweight mirrors will likely require on-orbit adjustment of alignment and figure.