Failure of concrete under load occurs in three phases: 1) distributed microcracking, 2) development of a macrocrack through the coalescence of microcracks and 3) macrocrack widening. This process governs the mechanical response of the material and has a strong impact on the durability of the concrete and the structure in which it is used. The strength of concrete, typically determined by the macrocrack development, can be increased by densifying the matrix through the use of low water-to-cement ratios and supplementary cementitious materials. However, this occurs at the expense of ductility, which is derived from energy-absorbing, distributed cracking. Conventional macrofibre reinforcement, with fibres greater than 0.5 mm in diameter, can slow the opening of macrocracks and impart post-peak ductility to the material. However, the presence of macrocracks provides avenues for the ingress of water and aggressive water-borne agents that can limit durability of concrete. Microfibre reinforcement, with fibre diameters less than 0.03 mm, slows the process by which microcracks evolve into macrocracks and so reduces permeability. To maximize the ductility and durability of concrete, crack development must be controlled. A hybrid blend of micro- and macrofibres is one method to achieve this control.