This book develops a comprehensive theoretical and empirical framework explaining how entrepreneurial ecosystems (EEs) influence firms' sustainable high growth (SHG). Its core objective is to conceptualize SHG as a key outcome of entrepreneurial ecosystems and to empirically verify whether, and through which mechanisms, ecosystems foster sustainable firm expansion. The theoretical part is based on a narrative literature review covering entrepreneurial ecosystems, firm growth, and growth determinants. It integrates ecosystem thinking with firm-level perspectives, including the resource-based view, and advances theory by defining the output of EEs as sustainable high-growth entrepreneurship-growth that combines size dynamics, profitability, and social benefits. Methodologically, this book makes a strong contribution by applying a quantitative, large-scale approach that remains rare in EE research. Using firm-level microdata for over 43,000 enterprises and regional data for Polish ecosystems from 2018-2022, it employs panel logistic regression, marginal effects analysis, Bayesian regression, and spectral clustering. Entrepreneurial ecosystems are operationalized through six dimensions and the study also captures firm heterogeneity, prior growth, and spatial interactions between neighboring ecosystems. Practically, the findings offer valuable insights for policymakers and regional development practitioners. By identifying ecosystem configurations most conducive to SHG, this book provides evidence-based guidance for designing targeted entrepreneurship and regional policies that promote productive, sustainable firm growth.