Protein kinase C (PKC) is a phospholipid-dependent protein kinase that plays important roles in various cellular functions. At least 11 PKC isozymes are known, and structural differences classify these isozymes into three groups, conventional PKC (cPKC), novel PKC (nPKC) and atypical PKC (aPKC). PKC isozymes exhibit differential tissue expressions and enzymatic properties. PKC may also play some roles in pathologic conditions. Several studies have shown the crucial roles of PKC isozymes in the transformation and growth of tumor cells, although these studies failed to show consistent results yet. In Alzheimer's disease, several studies revealed the special and systemic involvement of PKC beta isozyme.