The present invention relates to a novel class of histone deacetylase inhibitors with aryl-pyrazolyl motifs. The compounds of this invention can be used to treat cancer. The compounds of this invention are suitable for use in selectively inducing terminal differentiation, and arresting cell growth and/or apoptosis of neoplastic cells, thereby inhibiting proliferation of such cells. Thus, the compounds of the present invention are useful in treating a patient having a tumor characterized by proliferation of neoplastic cells. The present invention further provides pharmaceutical compositions comprising the compounds of this invention and safe dosing regimens of these pharmaceutical compositions, which are easy to follow, and which result in a therapeutically effective amount of the compounds of this invention in vivo.