This thesis summarizes experimental work using high pressure as a tuning parameter in the study of selected correlated electron systems at low temperatures. This thesis contains 8 chapters and 2 appendices. After giving the introduction in Chapter 1, experimental techniques are discussed in Chapter 2. Chapters 3 and 5 are devoted to pressure studies of ferromagnetic materials. In ferromagnetic systems, the appearance of quantum criticality is avoided either by a change of the second order transition to one of the first-order at a tricritical point or by the appearance of modulated, antiferromagnetic, phase. Chapter 3 summarizes the pressure study of the itinerant ferromagnet LaCrGe3 which reveals a new possibility for the phase diagram of metallic quantum ferromagnets. Chapter 4 presents a pressure study of a Ce-based ferromagnet, Whereas many Ce-based compounds manifest an antiferromagnetic ground state, only very few systems are known with ferromagnetic order and pronounced Kondo effects. CeTiGe3 is a rare example of a Ce-based ferromagnet. Chapter 4 presents our observation of a quantum tricritical point in the temperature-pressure-magnetic field phase diagram of CeTiGe3.

Chapters 5, 6 and 7 are devoted to pressure studies of superconducting materials. The results of PbTaSe2 are summarized in Chapter 5. PbTaSe2 is an interesting non-centrosymmetric superconductor which shows two superconducting regions separated by an exceptionally pressure sensitive structural phase transition. Chapter 6 discusses the non-monotonic pressure evolution of the superconducting upper critical field of FeSe. Our analysis of the evolution of the upper critical field under pressure provided evidence of a change of the Fermi surface under pressure in FeSe. In Chapter 7, the newly discovered CaKFe4As4 superconductor shows a collapsed tetragonal phase transition under pressure. In CaKFe4As4, we find that this is a very specific type of collapse, so-called half collapsed, with As-As bonding take place across the Ca-layer but not across the K-layer. This is the first observation of the half-collapsed-tetragonal phase transition in a Fe-based superconductor.