The human colonic microbiota exerts a profound influence on health, mediated by highly specific relationships among dietary fiber structures and microbial degraders. Systematic control of fiber structure offers opportunities to engineer microbiota-targeted interventions with increasing precision. Here, we examine the evolution of designed dietary fibers — biotechnologically produced oligosaccharides or polysaccharides synthesized de novo or naturally occurring polysaccharides intentionally modified post-extraction for their fine physical or chemical structures to influence gut microbiome. We propose a hierarchical framework to classify these fibers based on the degree of fine structure control, highlight current strategies for carbohydrate modification approaches, and discuss emerging directions for the field. Despite recent advances, much potential remains unrealized for the rational, reproducible design of microbiome-targeted fibers.