Biodiversity science is fundamentally structured around the concept of species, yet the definition, delimitation, and recognition of species remain among the most contested issues in biology. Over recent decades, advances in molecular biology, phylogenetics, ecology, and computational modeling have profoundly reshaped taxonomic practice, giving rise to integrative taxonomy as a dominant paradigm. This article presents a comprehensive and theoretically grounded examination of integrative taxonomy and species delimitation, synthesizing insights from molecular systematics, morphological analysis, ecological differentiation, biogeography, and scenario-based biodiversity modeling. Drawing exclusively on established literature, this study explores how different species concepts influence biodiversity assessments, how molecular techniques have transformed evolutionary inference, and how integrative approaches mitigate the limitations of single-data frameworks. Particular emphasis is placed on plant, fungal, algal, cyanobacterial, and animal case studies that demonstrate the practical consequences of species delimitation choices for conservation planning, ecosystem service modeling, and global biodiversity assessments. The article further interrogates the epistemological status of species as hypotheses, the role of coalescent theory and multilocus data in resolving complex evolutionary histories, and the implications of taxonomic uncertainty for large-scale biodiversity scenarios such as those developed by IPBES. By providing an exhaustive theoretical elaboration and critical analysis, this work positions integrative taxonomy not merely as a methodological toolkit but as a conceptual synthesis essential for robust biodiversity science in the Anthropocene.