Book excerpt: One of the biggest discoveries in condensed matter physics in the last couple of decades is the relationship between topology and condensed matter physics. On one hand, topology is a branch of mathematics that studies the basic properties of shapes and their spatial relations. On the other hand, condensed matter physics uses quantum mechanics to understand the macroscopic and microscopic behavior of solids. It turns out that these two seemingly different fields of knowledge are intertwined and deeply connected. This makes, the effort of understanding the consequences of topology in condensed matter systems a worthy scientific endeavor that will deepen our understanding of the basic properties of solids and might impact the future of technology in different applications. In this dissertation, we study the role of spin in the macroscopic behavior of novel families of topological quantum materials. We part ways from well-studied topological insulators and turn our attention to topological Dirac and Weyl semimetals. This work focuses on archetypal members of these families of materials namely: Cd3As2, TaAs and NbAs. We establish their synthesis in thin film form using molecular beam epitaxy (MBE) in common semiconductor substrates (GaAs, GaSb). We combine them with the soft ferromagnet, permalloy (Ni0.80Fe0.20), and use well established techniques such as spin torque ferromagnetic resonance (ST-FMR) and spin pumping (SP) to show that spin dependent phenomena plays an important role in these systems. This allows us to quantify the efficiency of the charge-spin interconversion in these materials, which together with electrical transport measurements allow us to estimate their spin Hall conductivity ([sigma]SH). These values are then compared with first principles calculations with a good qualitative and quantitative agreement. One of our main findings is that natural surface oxidation of these compounds plays a major role in spin transport and can enhance the charge-spin interconversion efficiency. With this knowledge, we take some members of these families of materials to the ultrathin regime. We have interfaced a Dirac semimetal (ZrTe2) with a two-dimensional magnet (CrTe2) and made a proof of concept device which shows that the spin of electrons in the Dirac semimetal can be controlled by electrical means and can be used to switch the magnetization direction of the ferromagnet, effectively making a quasi two-dimensional memory with topological materials. This motivated us to do similar experiments and study the behavior of spin in metals (Pb) once they are taken to the two-dimensional regime. This dissertation studies topology, spin transport and ferromagnetism in Dirac and Weyl semimetals, two-dimensional metals and two-dimensional ferromagnets.