Abstract: In this essay, we highlight some challenges the atmospheric community is facing concerning adequate treatment of flows over mountains and their implications for numerical weather prediction (NWP), climate simulations and impact modeling. With recent increases in computing power (and hence model resolution) numerical models start to face new limitations (such as numerical instability over steep terrain). At the same time there is a growing need for sufficiently reliable NWP model output to drive various impact models (for hydrology, air pollution, agriculture, …). The input information for these impact models is largely produced by the boundary layer (BL) parameterizations of NWP models. All known BL parameterizations assume flat and horizontally homogeneous surface conditions, and their performance and interaction with resolved flows is massively understudied over mountains – hence their output may be accidentally acceptable at best. We therefore advocate the systematic investigation of the so-called ‘Mountain Boundary Layer’ (MoBL), introduced to emphasize its many differences to the BL over flat and horizontally homogeneous terrain. An international consortium of scientists has launched a research program (TEAMx: ‘Multi-scale transport and exchange processes in the atmosphere over mountains – programme and experiment’) to address some of the most pressing scientific challenges. TEAMx is endorsed by World Weather Research Programme (WWRP), and the Global Energy and Water Exchanges (GEWEX) project as a ‘cross-cutting project’. A program coordination office was established at the University of Innsbruck (Austria). This essay introduces the background to and content of a recently published White Paper outlining the key research questions of TEAMx.