Genetic imprinting does occur in humans. This is manifest at the level of the genome, the individual chromosome or subcromosomal region, and the single locus. Genomic imprinting is manifest in the developmental defects of hydatiform mole, teratoma and triploidy. Chromosomal imprinting effects are revealed when uniparental disomy or deletion occur as costitutional aberration or in tumural tissues. Evidence at the single gene level comes from an increasing number of autosomal dominant genetic diseases. This part of the paper will briefly review the importance of imprinting in the contest of human genetic diseases.