Dietary fiber plays diverse physiological roles, including regulation of gastrointestinal function, metabolic health, immune responses, and disease prevention. This chapter outlines the historical evolution of dietary fiber definitions and highlights the limitations of traditional solubility-based classifications. A structure–function-based framework is proposed to better capture the complexity of fiber behavior in the human body, including properties such as fermentability, viscosity, and molecular structure. Furthermore, this chapter introduces recent research on novel soluble fibers with promising health benefits. In particular, agaro-oligosaccharides (AOS) and partially hydrolyzed guar gum (PHGG) have gained attention for their ability to modulate the gut microbiota, enhance short-chain fatty acid production, and exert anti-inflammatory and systemic effects. These findings support the potential of AOS and PHGG as high-value functional food components. Collectively, the chapter underscores the growing importance of dietary fiber in modern nutrition and its application in health-promoting interventions.