Dense hyphal mats formed by ectomycorrhizal (EcM) fungi are prominent features in Douglas-fir (Pseudotsuga menziesii) forest soils and have been estimated to cover up to 40% of the forest floor in some stands. Although previous studies have examined various aspects of EcM fungi, little is known about their associated microbial communities and activities. The objectives of my dissertation are (1) to provide a current account of chemical and biochemical properties associated with EcM fungi, and (2) to describe the communities and activities of microbes occupying EcM mat and non-mat forest soils. In the first phase of my research, I surveyed EcM mat and non-mat soils from early and late seral forest stands in the western Oregon Cascades. EcM mats were phylotyped and a variety of chemical and biochemical properties were measured. Results from this survey revealed distinct chemical and biochemical profiles for EcM mats in organic and mineral soils compared with their corresponding non-mat soil horizons. The second phase of my research followed up this work by focusing on Piloderma mats in old-growth Douglas-fir stands. A combination of community fragment profiles, clone libraries, and quantitative PCR of bacterial 16S and fungal ITS rRNA genes, in conjunction with chitinase enzyme assays, were used to assess the microbial community composition, abundance, and activity in a year-long temporal study. I found that Piloderma mats harbor distinct fungal and bacterial communities compared with non-mat soils. Furthermore, although microbial populations and enzyme activity of both soil types fluctuated throughout the year, their community compositions remained relatively stable. The results presented in this dissertation demonstrate that EcM mats create a unique soil environment with distinct microbial communities and activities compared to non-mat forest soils. This work provides a significant contribution to the understanding of how EcM fungi impact the soil environment and microbial communities.