Hepatitis A virus is an enteric picornavirus. Its genome is a single stranded RNA molecule of positive-strand polarity of 7478 bases. This sequence codes for a polyprotein which is processed to give rise to viral proteins VP-1, VP-2, VP-3 and others. Hepatitis B virus, a major worldwide infectious and cancer promoting agent contains a DNA genome of 3226 base pairs that replicates by a reverse transcriptase via an RNA intermediate. Extensive sequencing and expression experiments have revealed four major genes named surface, core, polymerase and X which are coded in more than one reading frame. Furthermore, within a frame, proteins are expressed from multiple initiation codons resulting in several related products. The viral genome of hepatitis C virus (nonA-nonB), an elusive major infectious agent, has recently been cloned. This genome is a single positive-stranded RNA of at least 10,000 bases which codes for several antigens, some of them associated specifically with nonA-nonB hepatitis infections. The hepatitis D (delta) viral agent, an infectious agent requiring a hepadnarious for propagation, contains a covalently closed circular single-stranded RNA genome of 1167 nucleotides. This genome encodes the protein p24 and p27 that bind specifically to antisera from patients with chronic hepatitis D infections.