This chapter gives an overview on tachycardias, which is also known as tachyarrhythmia. Tachycardia is a common type of heart rhythm disorder (arrhythmia) in which the heart beats faster than normal while at rest. In recent times, Vereckei has proposed a new algorithm in the differential diagnosis of wide QRS complex tachycardia which has been claimed to have a greater sensitivity and predictive value for ventricular tachycardia (VT) diagnosis and greater specificity and predictive value than those of Brugada criteria. Management of patients with atrioventricular nodal reentry tachycardia (AVNRT), wide complex tachycardia, pharmacological aids for the diagnosis of wide QRS tachycardia, recommendations of electrophysiological testing in patients with coronary heart disease, prevention and management of sudden cardiac death associated with acute coronary syndrome: esc guideline 2016, ventricular arrhythmias in hypertrophic cardiomyopathy, etc. are also discussed in this chapter. 17 well-illustrated flowcharts are also provided in this chapter.