In this entry, we have showcased the neutron techniques presently being used to study the flux-line lattice (FLL) in Type II superconductors. Due to the crystallisation of the FLL with a mesoscopic length scale, the workhorse technique of small-angle neutron scattering (SANS) continues to be the choice probe for studying FLLs and their properties in many superconducting classes, ranging from conventional superconductors such as elemental Nb (Laver et al., 2006), (Mühlbauer et al., 2009), to more exotic systems such as borocarbides (Yaron et al., 1996), (Eskildsen et al., 1998a), High-T_c cuprates (Cubitt et al., 1993), (Chang et al., 2012), pnictides (Eskildsen et al., 2011) and heavy-fermion materials (Bianchi et al., 2008), (Gannon et al., 2015). As an experimental probe of FLLs, SANS has a special status; in contrast to surface-sensitive techniques, SANS is a non-perturbative bulk probe of the FLL that allows a direct imaging of the FLL coordination and alignment. In favourable situations, information concerning the superconducting gap function of the material can be obtained. In addition, SANS studies can also be conducted for a wide range of applied magnetic fields ranging from typical values for H_c1 up to 17 T. This field range overlaps with, and even expands beyond those accessible by other probes of the FLL, enabling SANS to provide unique and complementary information concerning the microscopic FLL structure and perfection both in the bulk, and under extreme conditions. [...]