Fungi play pivotal roles in ecosystems by forming diverse associations with other organisms, including plants, animals and microorganisms, along with certain bacteria. These interactions range from mutualistic relationships, such as mycorrhizal associations facilitating nutrient uptake in plants, to parasitic interactions causing disease in the hosts. Fungi also engage in commensal relationships, where one organism benefits without harming or significantly benefiting the other. The ecological importance of these associations is profound, influencing nutrient cycling, biodiversity, and ecosystem stability. For instance, lichens, composed of fungi and algae or cyanobacteria, are pioneer species in harsh environments and contribute to soil formation. Understanding the dynamics of fungal interactions with other organisms is crucial for ecosystem management and conservation efforts. Moreover, exploring the molecular mechanisms underlying these associations can offer insights into biotechnological applications, such as agriculture and medicine, harnessing the potential of fungi for sustainable solutions in various fields. Fungi are eukaryotic microorganisms and can occur as yeasts, molds or as a combination of both forms. Mycology refers to the branch of biology involved in the study of fungi. It is concerned with the study of genetic and biochemical properties of fungi, as well as their taxonomy, benefits and harmful effects. There are three ways of classifying fungi based on their ability to reproduce asexually, sexually or by a combination of both. Fungi have several applications in a variety of industries such as food and pesticide. Yeast is a unicellular fungus used for making bread and other food items. The antibiotic known as penicillin is also obtained from a fungus named Penicilliumnotatum. This book provides significant information on fungi to help develop a good understanding of its diversity, ecology and biotechnological applications. It will also provide interesting topics for research which interested readers can take up.