Dislocation phenomena in crystal materials are fundamental to understanding mechanical behaviors such as plasticity and strengthening. This study investigates the atomistic mechanisms of dislocation phenomena in hexagonal close-packed (HCP) magnesium (Mg) and body-centered cubic (BCC) high entropy alloys (HEAs) using computational methods. We first examine dislocation motion in pure Mg, emphasizing prismatic slip mechanisms at different temperatures. Employing a range of neural network potentials (NNPs), we find that Mg exhibits an instability characterized by a potential-dependent critical stress in prism slip at low temperatures (below ~ 150 K). Three-dimensional simulations reveal a transition from cross-slip onto the basal plane at low stresses to prismatic loop expansion at higher stresses, consistent with in-situ TEM observations. This instability elucidates the origins of jerky low-temperature prismatic slip in Mg. For temperatures above ~ 150 K, we propose a new mechanism for prism glide, supported by theoretical analysis and molecular dynamics (MD) simulations. This intrinsically 3D mechanism involves the nucleation of a single kink at the junction of screw and non-screw characters in a prismatic dislocation loop, addressing the discrepancy in the glide barrier between experimental derivations and previous simulation results. The predictions of dislocation velocity and macroscopic strength align well with experimental data. Next, we explore dislocation phenomena in HEAs, focusing on edge dislocation strengthening in BCC HEAs. The effect of short-range order (SRO) on HEA strength is investigated using a recently developed SRO strengthening theory. Atomistic simulations indicate that alloy strength due to solute-dislocation interactions can vary depending on the SRO, consistent with theoretical predictions. These findings reveal the unexpected possibility of reduced strength due to SRO and validate the analytical theory as a tool for guiding alloy design. We then develop a strengthening theory for BCC edge dislocation slip on {112} planes, paralleling a recent theory for {110} slip. Using the atomistic dislocation pressure fields for four BCC elements (Nb, Ta, Mo, W) as proxies to span the range of likely alloy cores, the theory predicts similar strengths for {110} and {112} slip at finite temperatures and strain rates. This similarity confirms the applicability of the {110} edge theory for guiding alloy design, regardless of the actual slip system. Additionally, we propose a theoretical framework for phase-field modeling of edge dislocations in HEAs based on the Peierls-Nabarro model, incorporating complex solute effects. The phase-field method accurately captures the energy landscape encountered by dislocations during slip, demonstrating good agreement with atomistic nudged elastic band (NEB) calculations. This approach offers a computationally efficient method for studying dislocations at scales beyond direct atomistic simulations, allowing independent investigation of strengthening parameters. Through comprehensive simulations and theoretical analysis, this thesis advances the understanding of dislocation phenomena. The findings provide valuable insights into the mechanisms governing dislocation behavior and their impact on material properties in HCP magnesium and BCC HEAs, guiding the design of new materials with enhanced mechanical performance.