One of the most efficient methods for generating laser light in the 3–5 µm mid-infrared wavelength band involves the use of an optical parametric oscillator (OPO) to convert an incident pump photon into two longer-wavelength photons. By using a pump laser at a wavelength around 2 µm, both of the output photons from the OPO can be in the 3–5 µm band, greatly improving the conversion efficiency. Due to the nonlinear nature of the conversion process, a high-peak-power pump laser is essential for overcoming the large OPO threshold intensity, with pulsed lasers and amplifiers based on Ho:YAG being particularly attractive for such operation due to a natural emission peak at 2090 nm (where the absorption losses of OPO crystals are generally much lower than for shorter wavelengths) and a large energy storage capability. Ho:YAG sources are often pumped by continuous-wave thulium-doped fibre lasers in order to target the 1907 nm absorption line of the Ho:YAG crystals. However, all three stages (thulium fibre, Ho:YAG and OPO) suffer from a variety of intrinsic challenges that limit their operation at high average power levels, which have been investigated here.A novel dopant distribution profile has been explored within the core region of a cladding-pumped thulium fibre laser in order to permit much longer device lengths to be utilised and greatly improve the thermal management at high power levels, without being hampered by the onset of long-wavelength parasitic lasing before 1907 nm emission can be achieved. Up to 131 W of continuous-wave 1907 nm output is demonstrated from a single 6-metre-long section of nested-ring thulium-doped fibre, with the potential for adapting the nested-ring design to target other wavelengths that are limited by parasitic lasing also being examined. A high power master-oscillator power-amplifier Ho:YAG source has been developed using a 23.7 W, 36 ns Q-switched Ho:YAG seed laser which is injected into a two-stage amplifier system. Two 160 W all-fibre 1907 nm thulium pump sources were developed to single-end-pump the two amplifier crystals and greatly reduce the system complexity in comparison to alternative 1907 nm pump sources, generating up to 85 W of 2.1 µm output. The power scaling of radially polarised light at 2.1 µm has also been explored using Ho:YAG sources, including a 33.7 W double-pass thin-slab amplifier where the onset of depolarisation has been particularly targeted for mitigation. A 20.6 W radially polarised laser has also been demonstrated which uses an intra-cavity laser-written S-waveplate device to allow the simultaneous generation of radial polarisation and cavity Q-switching with up to 0.5 mJ pulse energy and a 210 ns pulse duration. Lastly, the potential advantages of using a highly-elliptical pump beam in a mid-infrared OPO are explored, initially by pumping the OPO directly with the 23.7 W Q-switched Ho:YAG seed laser, generating up to 3.7 W of mid-infrared output power for a 13:1 aspect ratio pump spot. The opportunity for further OPO power scaling when pumped with the 85 W MOPA output is also considered.