This chapter provides an introduction to mass spectrometry, covering the fundamental concepts and techniques used in the field. It begins by explaining the basic principles of mass spectrometry, including the interpretation of mass spectra, the calculation of isotopic percentages, and the understanding of fragmentation mechanisms. The chapter then delves into the ionization techniques, such as electron impact (EI), chemical ionization (CI), and mild electrospray ionization (ESI), detailing their mechanisms and applications. It also discusses the development of mass spectrometry over time, highlighting key milestones and advancements. The chapter further explores the interpretation of mass spectra, explaining how to identify molecular ions, isotopes, and fragments. It provides rules for interpreting peak heights, the nitrogen rule, and the identification of illogical peaks. The text also covers the fragmentation processes of various organic compounds, including alkanes, alkenes, cycloalkanes, halogenated compounds, alcohols, phenols, ethers, carbonyl compounds, carboxylic acids, nitrogen compounds, sulfur compounds, heterocycles, and biomolecules. Each section includes simplified spectra to aid in understanding the complex phenomena involved. Finally, the chapter discusses the acquisition of mass spectra, the importance of mass spectrometry in qualitative and quantitative analysis, and the use of derivatization techniques for challenging compounds. It concludes with a detailed analysis of the mass spectra of main classes of organic compounds, providing insights into their structural characteristics and fragmentation patterns.This chapter provides an introduction to mass spectrometry, covering the fundamental concepts and techniques used in the field. It begins by explaining the basic principles of mass spectrometry, including the interpretation of mass spectra, the calculation of isotopic percentages, and the understanding of fragmentation mechanisms. The chapter then delves into the ionization techniques, such as electron impact (EI), chemical ionization (CI), and mild electrospray ionization (ESI), detailing their mechanisms and applications. It also discusses the development of mass spectrometry over time, highlighting key milestones and advancements. The chapter further explores the interpretation of mass spectra, explaining how to identify molecular ions, isotopes, and fragments. It provides rules for interpreting peak heights, the nitrogen rule, and the identification of illogical peaks. The text also covers the fragmentation processes of various organic compounds, including alkanes, alkenes, cycloalkanes, halogenated compounds, alcohols, phenols, ethers, carbonyl compounds, carboxylic acids, nitrogen compounds, sulfur compounds, heterocycles, and biomolecules. Each section includes simplified spectra to aid in understanding the complex phenomena involved. Finally, the chapter discusses the acquisition of mass spectra, the importance of mass spectrometry in qualitative and quantitative analysis, and the use of derivatization techniques for challenging compounds. It concludes with a detailed analysis of the mass spectra of main classes of organic compounds, providing insights into their structural characteristics and fragmentation patterns.