This book aims to develop high school and undergraduate students' covariational reasoning and algebraic skills to succeed in calculus and STEM subjects.  The book reflects on contemporary research in math education where students explore algebraic tools and reason mathematically to construct new knowledge. The volume is made up of six chapters covering polynomial, rational, and transcendental functions. An early introduction of limits to support the analyses of linear functions progresses to other book chapters ensuring consistency, parallelism, and a scaffold knowledge delivery.  A gradual introduction to function rates of change along with function monotonicity and concavity intertwines with modeling techniques that merge students' mathematical reasoning with scientific contexts.  A forthcoming online component of the book consists of ready-to-download exploratory modeling activities and worksheets that further solidify students' fluency in understanding how to apply abstract math concepts to gain a deeper understanding of natural and social sciences.