This chapter overviews free radicals, highly reactive chemical species possessing unpaired electrons. The discussion encompasses free radicals’ nature, formation, and significance in various chemical and biological contexts. Free radicals are essential in physiological processes such as cellular signaling and immune response. However, their unchecked presence can lead to oxidative stress and biomolecule damage, contributing to aging and disease. Understanding the properties and behavior of free radicals is crucial for developing strategies to manage their effects and utilize them beneficially in fields including medicine and materials science. This chapter serves as a concise introduction to the multifaceted realm of free radicals, highlighting their importance and implications across scientific disciplines.