Suspensions of filamentous materials, or filamentous suspensions, represent a unique class of complex fluids in which the interplay between particle anisotropy and filament interactions – both intra- and inter-filament associations –gives rise to rich and tunable rheological behaviors. Such suspensions are ubiquitous across natural and engineered systems, with their applications ranging from the locomotion of microorganisms and drag reduction in turbulent flows to fiber processing in papermaking and the development of functional nanomaterials. Their characteristics of a filamentous or fibrillar nature, marked by a high aspect ratio particle structure and finite mechanical flexibility, enable not only the formation of entangled networks in which mechanics and dynamics depend strongly on dimension, concentration, and surface chemistry, but also bring greater complexity to material responses and flow phenomena. Despite their widespread utilization as colloidal materials, understanding how filamentous suspensions are affected by interactions among molecular species and filaments under varying conditions (pH, temperature, ionic strength, etc.), as well as mechanisms governing clogging, remains limited. Such understanding is crucial for tailoring their properties and performance to meet the needs of specific applications, such as rheology modification of complex fluids. To address these knowledge gaps, two complementary studies—rheology and clogging—are performed independently at the microscopic and macroscopic scales, respectively, to elucidate how the filamentous structure and associated physical properties manifest across multiple length scales. First, we systematically investigate the rheology of cellulose nanofibril (CNF) suspensions in the presence of salts and polymers to probe microscopic mechanisms arising from fibril-fibril, fibril-salt, and fibril-polymer interactions. Our studies reveal how ion–colloid and polymer–colloid interactions dictate the formation, structure, and dynamic response of CNF networks, providing a framework for enhanced CNF-containing complex fluid formulations. Secondly, we examine the clogging behavior of suspensions of threadlike colloids to uncover the macroscopic mechanisms underlying collective dynamics in these granular systems. Analysis of clogging probability and flow conditions provides insight into the transition between flow and jammed (clogged) states. Together, these investigations establish a deeper understanding of how anisotropic filamentous geometry and its corresponding interactions govern complex rheological and flow-arrest behaviors, thereby guiding the design of advanced materials and the optimization of processing for filament-based materials.