In this thesis, the results of four electronic transport studies of correlated electron systems performed under hydrostatic pressure, at low temperatures and in the presence of high magnetic ﬁelds are presented. Firstly, magnetoresistance and Hall eﬀect measurements in Tl2Ba2CuO6+δ, an overdoped cuprate, are presented. At low magnetic ﬁelds, a comparison between the evolution of the in-plane transport properties with a suppression of the superconducting transition temperature tuned by both chemical doping and the application of hydrostatic pressure. At high magnetic ﬁelds and in a region of doping-temperature phase-space in which conventional transport theory has previously been deployed to accurately describe the transport properties of highly overdoped cuprates, a highly unconventional magnetoresistance has been revealed. Deﬁnitive evidence of two charge sectors is presented where the ﬁrst is orbital in nature whilst the second is highly unconventional and likely incoherent. Secondly, a high-ﬁeld Hall eﬀect study of YBa2Cu3O7−x, an underdoped cuprate, is presented. The evolution of the superconducting transition temperature and the temperature at which the Hall coeﬃcient changes sign, an indicator of a Fermi surface reconstruction due to the onset of charge order, are tracked as a function of hydrostatic pressure. The ﬁrst is used as a measure of the strength of superconductivity whilst the second is used as a proxy for the strength of charge order. The implications with regard to the interplay or competition between the two phases are discussed. Thirdly, a comprehensive study is presented of the in-plane magnetoresistance under pressure of FeSe1−xSx with sulphur concentrations that span the nematic phase. Nematicity is suppressed with pressure. At the critical pressure Pc at which the nematic transition is suppressed to zero temperature (at the nematic QCP), the in-plane resistivity measured at 35T exhibits hallmarks of quantum criticality. Speciﬁcally, the high-ﬁeld resistivity is highly linear down to lowest temperatures and a growing region of T2 behaviour is observed at lowest temperatures as P is increased beyond Pc. Finally, a quantum oscillation study of the nodal-line semi-metal ZrSiS has been performed under hydrostatic pressure. At ambient pressure, unconventional breakdown phenomena have been observed. Analysis of the Shubnikov de-Haas oscillations at both ambient and elevated pressures has been performed to elucidate the nature of the Fermi surface and the origin of the aforementioned breakdown phenomena.