Heart rate variability (HRV) is a non-invasive marker of cardiac autonomic regulation that reflects the dynamic interaction between sympathetic and parasympathetic influences on the sinoatrial node. Reduced HRV is a hallmark of autonomic dysfunction and has been consistently associated with adverse outcomes across a wide range of cardiovascular conditions, particularly heart failure (HF). HRV is markedly reduced in HF, reflecting profound autonomic imbalance characterized by vagal withdrawal, impaired baroreflex sensitivity, and altered cardiorespiratory coupling. In heart failure with reduced ejection fraction (HFrEF), lower global HRV, particularly reduced SDNN, is consistently associated with mortality, sudden cardiac death, and disease progression. In heart failure with preserved ejection fraction (HFpEF), emerging observational evidence suggests that HRV alterations may accompany early autonomic dysregulation and may be associated with sex-specific patterns, although their mechanistic and prognostic significance remains incompletely defined. In acute HF, HRV assessed during hospitalization may provide additional prognostic information, although current evidence remains limited and heterogeneous. Pharmacological therapies, cardiac resynchronization therapy, exercise training, and lifestyle interventions have variably modulated HRV across studies, but the evidence is heterogeneous and changes in HRV should not be assumed to translate directly into improved clinical outcomes. Despite persistent challenges related to methodological standardization and clinical implementation, emerging technologies and multimodal approaches may refine the feasibility of integrating HRV into risk assessment frameworks. This narrative review aims to summarize the physiological basis of HRV, critically evaluate its alterations across different HF phenotypes, and discuss its prognostic associations, therapeutic modulation, and current practical limitations in clinical application.