The chapter discusses the mechanisms of membrane toxicity of hydrocarbons, particularly cyclic hydrocarbons, on microorganisms. It highlights the importance of these compounds in nature and technological applications, such as wastewater treatment and bioremediation. The text reviews the metabolic pathways and genetic aspects of hydrocarbon degradation by microorganisms, noting that while these processes can lead to complete mineralization, the low stability of microbial activity due to cell inactivation is a significant challenge. The toxicity of hydrocarbons is attributed to their lipophilic nature, which interacts with the hydrophobic parts of the cell membrane. The chapter also covers the bioavailability of cyclic hydrocarbons, the dissolution of these compounds in water, and the impact of physical and chemical parameters on their solubility and toxicity. Additionally, it explores the cell envelope structure of microorganisms, including the cell wall, lipid membranes, and crystalline surface layers, and how these structures affect the uptake and partitioning of hydrocarbons. The toxicity of specific hydrocarbons, such as terpenes, aromatics, and cycloalkanes, is discussed, along with the mechanisms by which they disrupt cellular functions, such as energy transduction and membrane integrity. The chapter concludes with a discussion on the toxicity of other lipophilic compounds, including food preservatives, disinfectants, and drugs, and their effects on microorganisms.The chapter discusses the mechanisms of membrane toxicity of hydrocarbons, particularly cyclic hydrocarbons, on microorganisms. It highlights the importance of these compounds in nature and technological applications, such as wastewater treatment and bioremediation. The text reviews the metabolic pathways and genetic aspects of hydrocarbon degradation by microorganisms, noting that while these processes can lead to complete mineralization, the low stability of microbial activity due to cell inactivation is a significant challenge. The toxicity of hydrocarbons is attributed to their lipophilic nature, which interacts with the hydrophobic parts of the cell membrane. The chapter also covers the bioavailability of cyclic hydrocarbons, the dissolution of these compounds in water, and the impact of physical and chemical parameters on their solubility and toxicity. Additionally, it explores the cell envelope structure of microorganisms, including the cell wall, lipid membranes, and crystalline surface layers, and how these structures affect the uptake and partitioning of hydrocarbons. The toxicity of specific hydrocarbons, such as terpenes, aromatics, and cycloalkanes, is discussed, along with the mechanisms by which they disrupt cellular functions, such as energy transduction and membrane integrity. The chapter concludes with a discussion on the toxicity of other lipophilic compounds, including food preservatives, disinfectants, and drugs, and their effects on microorganisms.