Checkpoint kinase 2 (CHEK2, Chk2) emerges as an important signal transducer of cellular responses to DNA damage and a candidate tumor suppressor whose defects contribute to molecular pathogenesis of diverse types of human malignancies, both sporadic and hereditary. Here, we briefly outline the molecular properties, regulation and physiological role of CHEK2, and review in more detail its defects that predispose to tumors, with particular emphasis on familial breast cancer. The frequency, penetrance and epidemiological as well as clinical significance of the two most studied breast cancer-predisposing variants of the CHEK2 gene, 1100delC and I157T, are highlighted in more depth, and additional CHEK2 mutations and their cancer relevance are discussed as well. These recent findings are considered also from a broader perspective of CHEK2 as the integral component of the ataxia telangiectasia-mutated-CHEK2-p53 pathway within the genome integrity maintenance system and a barrier against tumor progression. Finally, the potential value of information about the CHEK2 status in family counseling and optimizition of individualized cancer treatment is discussed.