Rare diseases represent one of the most complex areas of modern biomedical research due to their low prevalence, marked clinical heterogeneity, multisystem involvement, diagnostic delays, and limited therapeutic options. Contemporary rare disease biology is evolving beyond traditional phenotype-based classifications toward integrated translational models that incorporate molecular endotypes, cellular network organization, immunometabolic processes, and dysregulated biological pathways. The observation that similar clinical phenotypes may arise from distinct molecular mechanisms, while different diseases may share common network perturbations, highlights the multilayered pathobiological nature of rare diseases. In this chapter, rare diseases are evaluated using a clinical system–based approach encompassing neurological and neurodevelopmental disorders, metabolic and lysosomal storage diseases, immunological and autoinflammatory disorders, chromosomal and genomic diseases, and mitochondrial disorders. Each disease is discussed in terms of associated genes, chromosomal localization, encoded proteins, molecular signaling pathways, cellular pathophysiology, clinical phenotypes, and current targeted therapeutic strategies. In addition, modern translational therapeutic approaches, including gene therapies, antisense oligonucleotide technologies, RNA-targeted strategies, and molecularly targeted biological agents, are addressed. In conclusion, modern rare disease classifications are being reshaped through the integration of systems biology, network-based medicine, and precision medicine approaches, thereby contributing to a more comprehensive understanding of patient-specific molecular mechanisms, biomarker discovery, and the development of targeted therapeutic strategies.