Heart failure with midrange ejection fraction (HFmrEF) is a new category of heart failure (HF), inbetween HF with reduced ejection fraction (HFrEF) and HF with preserved ejection fraction (HFpEF). Previous studies were mainly conducted in HFrEF patients having a left ventricle ejection fraction (LVEF) lower than 35-40%. Later on, HFpEF captured the spot-light of the research field, and studies focused on patients with HF symptoms, but with a LVEF exceeding 50%. Consequently, a gap of knowledge comprising the LVEF between 40 and 49% has arisen. Current studies focusing on patients with HFmrEF are arguing the same conclusions or even having contradictory findings. HFmrEF has a prevalence of 10-20% of HF patients. HFmrEF has distinct, but intermediate clinical, structural and functional characteristics, as well as intermediate outcomes in comparison with HFrEF and HFpEF. However, there is still a large gap in evidence regarding detailed hemodynamic characteristics, long-term follow-up and optimal therapeutic options for these patients. Extensive research was recommended in order to improve knowledge about this "gray area" of patients with HF. Therefore, we aimed to provide an over-view of the existing and lacking data regarding patients with HFmrEF.