Diffusion plays a critical role in biofilm growth, influencing the chemistry and biology of microbial communities. In biofilms, diffusion is the primary transport mechanism, unlike in free aqueous suspensions where convection dominates. Biofilms have increased diffusion distances and reduced fluid flow, leading to diffusion limitations that create spatially heterogeneous environments. This heterogeneity results in varying concentrations of substrates, products, and microbial species, affecting biofilm physiology and function. Diffusion times in biofilms depend on the biofilm thickness, the effective diffusion coefficient, and the geometry of the system. For example, a 100 μm thick biofilm may take about 165 seconds for a solute to diffuse through it. Similarly, a 260 μm radius spherical biofilm may require 298 seconds for a solute to penetrate. These times can be significantly longer than in free suspensions due to the increased diffusion distance in biofilms. Water channels in biofilms may facilitate solute transport but do not guarantee access to the interior of cell clusters. Biofilm structure and geometry influence diffusion, but the fundamental principles of diffusion remain unchanged. Diffusion limitations can lead to concentration gradients and physiological differences within biofilms, affecting microbial interactions and survival. While diffusion is a key factor in biofilm behavior, it does not explain all phenomena. Microorganisms may have alternative pathways for sensing and responding to their environment, such as quorum sensing, which is independent of diffusion-based concentration gradients. These pathways can lead to the development of protected phenotypes, such as persister cells, which are less susceptible to antimicrobial agents. In summary, diffusion is the dominant transport process in biofilms, leading to spatial heterogeneity and influencing biofilm chemistry and biology. Diffusion limitations create gradients that affect microbial interactions and survival, but other mechanisms, such as quorum sensing, may also play a role in biofilm development and function.Diffusion plays a critical role in biofilm growth, influencing the chemistry and biology of microbial communities. In biofilms, diffusion is the primary transport mechanism, unlike in free aqueous suspensions where convection dominates. Biofilms have increased diffusion distances and reduced fluid flow, leading to diffusion limitations that create spatially heterogeneous environments. This heterogeneity results in varying concentrations of substrates, products, and microbial species, affecting biofilm physiology and function. Diffusion times in biofilms depend on the biofilm thickness, the effective diffusion coefficient, and the geometry of the system. For example, a 100 μm thick biofilm may take about 165 seconds for a solute to diffuse through it. Similarly, a 260 μm radius spherical biofilm may require 298 seconds for a solute to penetrate. These times can be significantly longer than in free suspensions due to the increased diffusion distance in biofilms. Water channels in biofilms may facilitate solute transport but do not guarantee access to the interior of cell clusters. Biofilm structure and geometry influence diffusion, but the fundamental principles of diffusion remain unchanged. Diffusion limitations can lead to concentration gradients and physiological differences within biofilms, affecting microbial interactions and survival. While diffusion is a key factor in biofilm behavior, it does not explain all phenomena. Microorganisms may have alternative pathways for sensing and responding to their environment, such as quorum sensing, which is independent of diffusion-based concentration gradients. These pathways can lead to the development of protected phenotypes, such as persister cells, which are less susceptible to antimicrobial agents. In summary, diffusion is the dominant transport process in biofilms, leading to spatial heterogeneity and influencing biofilm chemistry and biology. Diffusion limitations create gradients that affect microbial interactions and survival, but other mechanisms, such as quorum sensing, may also play a role in biofilm development and function.