Bioavailability for contaminant degradation requires a deep understanding of the ecology of degrader microbial systems. It hence should be perceived as a microbial systems property. In this chapter we summarize recent research on microbial ecology of contaminant biodegradation in the mycosphere (i.e., the microhabitat surrounding and affected by mycelia). By forming unique transport networks, mycelial fungi are highly adapted to cope with complex heterogeneous habitats and to grow under conditions of uneven availability of their vital resources. Combining concepts from bioavailability, ecophysiology, and microbial ecology, our chapter discusses the impact of fungal networks on chemical and bacterial transport and their effects on contaminant bioavailability and degradation. It thereby provides generic information on key factors, processes, and ecological principles that drive contaminant biotransformation in the mycosphere.