Ectomycorrhizas are mutalistic symbiotic associations formed between fine roots of higher plants, mostly trees, and a wide range of soil ascomycetes and basidiomycetes. It is commonly accepted that there is mutual benefit to the partners, due to the exchange of plant-derived carbohydrates for amino acids and nutrients supplied by the fungus. While the major concepts of mycorrhizal functioning (exchange of nutrients and metabolites) were proposed in the 1960s, their verification at the molecular level started approximately 10 years ago. This review covers concepts at the molecular level concerned with the fungal carbohydrate supply in symbiosis. We discuss: strategies used by host plants to compensate (and perhaps restrict) carbohydrate drain to the fungal partner; fungal mechanisms that generate strong monosaccharide sinks in colonized plant roots (the formation of a strong carbohydrate sink is a prerequisite for efficient fungal carbohydrate support by the plant partner); and the impact of apoplastic hexose concentration on the regulation of fungal metabolism in symbiosis, since monosaccharides not only serve as nutrients but also as a signal that regulates gene expression.