A rapidly growing area in the field of materials science is the study of how materials respond under extreme compression. High pressure forces atoms into higher densities, significantly reducing interatomic distances and increasing orbital overlap. These conditions often lead to the formation of new and exciting structures that are not stable under ambient pressures, thereby leading to exotic new physical and chemical properties. New materials with unique properties like ultrahardness, magnetism, or superconductivity are necessary to advance technology such as wind harvesting, high-speed transportation, advanced machining and drilling, and electricity distribution. For known materials, studying how the bulk properties respond to high pressure in the laboratory gives tremendous insight into how they will function under extreme conditions that arise in demanding applications. For example, understanding how steels and other alloys respond to elevated pressures and temperatures—and in particular under ultrafast conditions—is of special interest to those tasked with x safeguarding our nation’s stockpile of nuclear arms. In this dissertation work, I have used shockwaves to recreate ultrafast rates of pressure and temperature changes in the laboratory. I present two systems of interest studied at the Matter in Extreme Conditions instrument using the Linac Coherent Light Source at SLAC National Accelerator Laboratory. In both projects, my experimental goal was to study how the chemical bonding and crystallographic structure of the material changes upon compression to extreme conditions. I used in-situ X-ray diffraction to determine the precise crystallographic structures formed on a nanosecond timescale under shock conditions, and then mapped these phases and compare them to those observed with static measurements to quantify kinetic effects arising in the ultrafast regime. I also used the velocity interferometry system for any reflector (VISAR) as a method to characterize the particle velocity experienced by the sample and thereby back-calculate the peak stress experienced by the sample. My research on nickel has uncovered some surprising results. Nickel is a constituent of our earth’s core, occurring alongside iron at a ratio of 5-15%. While nickel has been well-characterized at low pressures, very little data exist at high pressures and temperatures. Through my analysis of the X-ray diffraction data, I have determined that nickel maintains the face-centered cubic structure up to ∼500GPa along the principal Hugoniot, or nearly twice the pressure at the center of our Earth. This is the first in situ X-ray diffraction data on shock-compressed nickel up to ∼500GPa and is particularly relevant to computational and experimental researchers studying nickel and other dense metals. The pressures I reached are some of the highest ever reported for shocked nickel, and my identification of a solid compressed phase up to 332GPa is significantly higher than expected by the majority of melt lines that have been proposed for nickel in the literature. The second project I discuss combines a novel sample preparation method with the first in situ X-ray diffraction study under shock compression on the manganese(II) oxide system. The samples were prepared in-house through a rolled slurry method that I designed. To address some of the limitations currently hindering study in this field, this method involves mixing a powder, such as manganese(II) oxide, with an epoxy matrix to produce a well-mixed slurry. This slurry can then be rolled into smooth sheets of sample with precisely controlled thicknesses. My goal was to reduce localized height and texture variances in the sample and to circumvent the need for additional glue layers whose thicknesses are often inconsistent and difficult to measure. These glue layers been shown to impact the quality of experimental data by making it difficult to precisely model shock transition times with hydrocode simulations. The ability to embed powders into an epoxy polymer matrix would allow us to test many materials that are not compatible with standard methods. Applying this method to samples of manganese(II) oxide, I have observed the formation of the B2 phase at ∼150 GPa. While the B2 phase is well-documented in other systems such as magnesium oxide, iron oxide, and nickel oxide, it has not been previously observed in manganese(II) oxide. My investigation combines VISAR data and in situ X-ray in order to map out the phase diagram of manganese(II) oxide under dynamic compression conditions.