This article provides a comprehensive overview of free radicals, including their properties, sources, and molecular targets, as well as their implications in various diseases. Free radicals, primarily reactive oxygen species (ROS) and reactive nitrogen species (RNS), are derived from both endogenous and exogenous sources and play dual roles in biology. While they have beneficial effects in moderate or low concentrations, such as in immune function and cellular signaling, excessive production can lead to oxidative stress and nitrosative stress, causing damage to biomolecules like nucleic acids, lipids, and proteins. This review highlights the chemistry, formation, and sources of free radicals, and discusses their involvement in diseases such as diabetes mellitus, neurodegenerative disorders, cardiovascular diseases, respiratory diseases, cataracts, rheumatoid arthritis, and various cancers. The article also details the molecular targets of free radicals, including DNA, RNA, lipids, and proteins, and explains how they contribute to disease progression.This article provides a comprehensive overview of free radicals, including their properties, sources, and molecular targets, as well as their implications in various diseases. Free radicals, primarily reactive oxygen species (ROS) and reactive nitrogen species (RNS), are derived from both endogenous and exogenous sources and play dual roles in biology. While they have beneficial effects in moderate or low concentrations, such as in immune function and cellular signaling, excessive production can lead to oxidative stress and nitrosative stress, causing damage to biomolecules like nucleic acids, lipids, and proteins. This review highlights the chemistry, formation, and sources of free radicals, and discusses their involvement in diseases such as diabetes mellitus, neurodegenerative disorders, cardiovascular diseases, respiratory diseases, cataracts, rheumatoid arthritis, and various cancers. The article also details the molecular targets of free radicals, including DNA, RNA, lipids, and proteins, and explains how they contribute to disease progression.