Humans have used mushrooms for centuries, serving as a vital food source and in traditional medicine, culinary practices, and cultural rituals. At the same time, their modern applications extend to pharmaceuticals, biotechnology, and sustainable industries. However, not all mushrooms are edible; some inedible mushrooms are also present and can be poisonous. The consumption of poisonous mushrooms has led to many health problems and even deaths throughout the world. Therefore, this chapter discusses mushrooms that should be avoided. In our study, we reviewed the comparison between inedible and edible mushrooms, as well as the taxonomy of the latter. Moreover, we have also discussed the toxic compounds in inedible mushrooms and their effects on living organisms. In addition to this, the medicinal and nutritional importance of inedible mushrooms is also provided. Finally, protective measures against mushroom poisoning are presented as precautions against mushroom toxicity.