Amphetamines or amphetamine-type stimulants (ATSs) refer to a group of pharmacological and -toxicological agents that have a common phenethylamine structural backbone and typically impart effects that include, but are not limited to, vasoconstriction, anorexia, central nervous system stimulation, and/or hallucinations. While differences in side chain chemistry can impart different pharmacological or toxicological effects, for some compounds, e.g., MDMA (Ecstasy), alterations of the phenyl part of the molecule impart other significant effects. ATSs are used both therapeutically and recreationally, with significant abuse and addiction potential. Therapeutically, these agents are mainly used to treat hyperactivity disorders or aid in weight loss. Toxicological effects include hypertension, arrhythmia, excitability, aggressiveness, psychoses, coma, and death.Traditional analytical methods to analyze amphetamines include gas chromatography-mass spectrometry where derivatization is often required to facilitate analysis. Besides sample preparation issues, it has been demonstrated that injection port chemistry in the GC can lead to misleading results with some members of the amphetamine class. To circumvent these issues, liquid chromatography-mass spectrometry (LC-MS/MS) offers the promise of a simpler sample preparation procedure and fewer analytical concerns. This chapter describes an LC-MS/MS technique for the analysis of 14 ATSs in blood, serum/plasma, and urine. The method is quantitative and has reporting limits in the low ng/mL range. Electrospray ionization is used in the positive ion mode. Two transitions for each compound are monitored along with ion ratios.