This application describes methods and apparatus for fibre optic distributed acoustic sensing (DAS) where microstructured fibre (202), such as holey fibre or photonic crystal fibre is used as the sensing fibre (104). The microstructured fibre is configured so to provide at least one of enhanced sensitivity to a given incident acoustic signal; an enhanced non-linear optical power threshold and directional sensitivity. By configuring the microstructured fibre to be more compliant than an equivalent solid fibre and/or provide a large refractive index variation in response to applied strain, the response to a given acoustic stimulus may be larger than for the equivalent fibre, Providing a hollow core may allow higher optical powers and by providing a directionality to microstructured (304) allows the fibre to be used in a DAS system with a directional response.