Modern scientific and engineering systems are increasingly defined by nonlinearity, high dimensionality, partial observability, and uncertainty. Addressing such complexity requires more than isolated techniques. It calls for an integrated applied mathematics perspective that connects modeling, data analysis, and computation within a coherent framework. This book develops such a unified toolkit. Rather than treating mathematical methods as separate disciplines, it emphasizes their complementary roles in understanding, analyzing, and predicting complex systems, with a focus on intuition, practical relevance, and transferability across scientific and engineering fields. Presents a broad collection of essential applied mathematics tools, organized to emphasize their interplay rather than treating them as isolated topics. Each method is introduced through simple yet illuminating examples that make advanced concepts accessible without sacrificing rigor. For every technique, the presentation highlights the underlying motivation, key assumptions, strengths, limitations, and the contexts in which it is most effective. Theory, data, and algorithms are consistently integrated to demonstrate how they work together in practical applications. Is accompanied by short ``one concept $\cdot$ one example'' videos, each 5-10 minutes long, focusing on a single idea and using deliberately simple examples to support initial exposure, intuition building, and efficient review. Example codes included to reinforce the concepts and facilitate hands-on learning. This book is intended for advanced undergraduate students, graduate students, and researchers in applied mathematics, science, and engineering who seek practical mastery of modern mathematical tools. It is well suited to topic courses and self-study, particularly for readers working at the intersection of modeling, data, and computation. By combining conceptual clarity with hands-on examples and an integrated methodology, the book provides a foundation for tackling real-world problems in a wide range of interdisciplinary settings.