In this review article about fibrosis and arrhythmias, we show that the amount of collagen, a normal element of the heart muscle, increases with age and in heart disease. The relation between fibrosis and electrophysiological parameters such as conduction, fractionation of electrograms, abnormal impulse initiation as well as arrhythmogenicity is discussed. Next to the amount of fibrosis, we offer data suggesting that collagen texture too plays a role in conduction slowing and arrhythmia vulnerability. Data are shown revealing that fibrosis can also be induced by reduced sodium channel and connexin43 expression. Finally contrast-enhanced magnetic resonance to detect fibrosis and ventricular tachycardia vulnerability in a noninvasive way as well as a reduction of fibrosis and arrhythmogenicity by inhibition of the renin-angiotensin-aldosterone system is discussed.