Only in the last decade has it been recognized that chronic kidney disease (CKD) and chronic renal failure represent potent cardiovascular risk predictors. With glomerular filtration rates below 60 ml/min, the risk of cardiovascular events rises steeply, but an increased prevalence of traditional risk factors cannot be held responsible for this observation. Therefore, a pathological and specific cardiorenal communication seems to be associated with progressive renal function loss. However, there is not just “crosstalk” from CKD to cardiovascular morbidity, but heart and kidney also interact with each other in any situation involving acute injury of each organ. This perception prompted a working group, with Ronco coining the term “cardiorenal syndrome” and developing a classification of this term into five categories. Although this classification still lacks full international and evidence-based approval, the possibility appears that it may develop into a clinically useful concept, which is outlined in this review.