The study of phase-separated metallic glasses (PSMGs) is motivated by both theoretical curiosity and their potential practical applications. It offers important understanding of the atomic arrangement and properties of disordered materials, while also giving new pathways for developing advanced materials with tailored properties. PSMGs exhibit unique structural and physical characteristics that distinguish them from monolithic metallic glasses (MGs), allowing for enhanced mechanical, thermal, and chemical performance. Recent advances in synthesizing PSMGs have paved the way for engineering materials with hierarchical microstructures across multiple length scales, enabling the design of novel composites with optimized strength, ductility, corrosion resistance, and other desirable properties. This chapter presents an in-depth exploration of nanoscale phase separation in MGs, emphasizing its influence on material properties. The discussion is organized into two main units. The first unit provides a brief overview to phase separation in MGs, discussing the underlying mechanisms, microstructural evolution, thermal behavior, and the advantages of phase-separated structures. It also highlights selected findings related to their morphology and properties. The second unit discusses recent advances in Zr-based PSMGs with respect to their design and properties. This also includes the new results of in-situ transmission electron microscopy (TEM) tensile deformation and corrosion tests of phase-separated Zr–Al–Fe–Y MG compositions, which have not been reported so far.