The present review is focuses on new onset atrial fibrillation (AF) in conditions of acute coronary syndrome (ACS). Literature sources from PubMed and Scopus was used. AF is common in the general population and in the ACS population, at that new onset AF, and AF, which was by the time of ACS development are determined. Appearance of AF is more likely amongst the elderly patients with congestive heart failure, with signs of the hemodynamic instability and with the left atrium dilatation. It is well-known about the predictors of adverse outcome in ACS. According to some literature data new onset AF leads to worsening of prognosis, and in line with other sources a reason of its association with high level of mortality is due to the severity of ACS and appearance of AF is not independent predictor of death. The absence of subjective symptoms at the time of paroxysm of new onset AF does not allow estimating its duration and distinguishing between the new onset AF, persistent and constant AF. It is possible to trace the relationship between the myocardial ischemia and AF appearance. Inefficient reperfusion during percutaneous coronary intervention or thrombolytic therapy is accompanied by the onset of AF, on the contrary, when effective patency of coronary artery is achieved, AF appears significantly less often. New onset AF in ACS, in particular, accompanied by cardiogenic shock, requires emergency pharmacological or electrical cardioversion. In some cases, active cardioversion is not necessary, because of spontaneous cardiac rhythm conversion. In conclusions, it is necessary to point out, that ambiguity and multifactority of this problem demands investigation of arrhythmogenesis mechanisms and development of special risk stratification instruments for the new onset AF in ACS.