Optical microresonators hold great potential for many fields of research and technology. However, due to their small dimensions typical microresonators exhibit a large frequency spacing between resonances and a limited tunability. This impedes their use in a large class of applications which either require a resonance of the microcavity to coincide with a predetermined frequency, e.g., an optical transition in atoms, or a tailored frequency spacing between resonances, e.g., for the generation of optical frequency combs. Here, we present a fully tunable ultra-high-Q whispering-gallery-mode “bottle microresonator”, fabricated from standard optical glass fibres. Due to its highly prolate shape, the bottle microresonator gives rise to a class of whispering-gallery-modes (WGMs) with advantageous properties, see Fig. 1. In addition to the radial confinement by continuous total internal reflection at the resonator surface, the light in these “bottle modes” oscillates back and forth along the resonator axis between two turning-points which are defined by an angular momentum barrier [1]. The resulting axial standing wave structure can be compared to the one observed in Fabry-Pérot microresonators.