A collection of studies that focus on applications of electron paramagnetic resonance (EPR) spectroscopy and superconducting quantum interference device (SQUID) magnetometry are reported. Part of these studies are dedicated to the investigation of the electronic structure and quantum coherence properties of a number of organometallic lanthanide and actinide complexes that display slow relaxation of the magnetisation at low temperature and thus single ion magnet behaviour. Other studies focus on the development of electron paramagnetic resonance instrumentation that operates at sub-Kelvin temperatures and subsequent application of this novel instrumentation to studies of high-spin paramagnetic complexes. In the first chapter, a combination of continuous-wave and advanced pulsed EPR methods are used to investigate the quantum coherence properties of two very unusual tris-cyclopentadienyl monometallic complexes based on yttrium(II) and lutetium(II), respectively. Long coherence times were measured in both compounds. This is unexpected due to the large number of 1H nuclei surrounding the electron spin. Remarkably, thermal dependence of the relaxation times and quantum Rabi oscillations could be studied up to room temperature in an electron spin diluted Y/Yb single crystal. The lutetium(II) derivative also shows quantum Rabi oscillations persisting up to 100 K, which are accessible for each of the eight electronuclear transitions originating from hyperfine interactions with 175Lu. Moreover, two-dimensional HYSCORE (hyperfine sublevel correlation) enabled pioneer measurements of actinide covalency in two isostructural cyclopentadienyl thorium(III) and uranium(III) complexes by measuring the hyperfine interactions with the ligand 13C and 1H nuclei. Chapter two reports on studies of pseudo-linear bis(methanediide) lanthanide(III) single-ion magnets whose remarkable properties are related to their strongly axial ligand environments. The compounds exhibit rich magnetisation dynamics that culminates with the observation of out-of-phase magnetic susceptibility maxima up to 42 K in a dysprosium(III) complex. Remarkably, slow relaxation of the magnetisation occurs via two thermally activated energy barriers of 721 and 813 K, respectively. The holmium(III) derivative also shows single ion magnet behaviour under zero applied dc field (Hdc = 0) accompanied by slow magnetic relaxation below 25 K. This behaviour is associated with a spin relaxation barrier of 344 K at zero dc field (Hdc = 0), which is the highest yet recorded for any mononuclear holmium(III) complex. Chapter three describes the development of 34 GHz resonator and a cryogen-free cryostat operating at temperatures below 1 K, compatible to be integrated with standard commercial EPR spectrometers. The functionality of the sub-Kelvin instrumentation was tested on a high-spin dodecametallic chromium(III) cage, which acts as a temperature calibrator. Pulsed EPR tests on the resonant cavity paves the way for further development of the system to enable mK pulsed EPR experiments.