The high transition temperature superconductors represent a new class of materials, which have extraordinary superconducting and magnetic properties and great potential for a wide-range of technological applications. Strong antiferromagnetic correlations, quasi-two dimensional anisotropy, d-wave pairing, very short coherence length, and strong fluctuations characterize these materials. All these characteristics strongly modify the behavior of flux vortices present in a superconductor placed in a magnetic field and their dynamics. As a consequence, the magnetic flux matter and its dynamics are unique in this class of materials. In this talk, I will show evidence of vortex excitations above the zero-field superconducting transition temperature through magetotransport measurements. The nature of vortex dissipation over a wide range of temperatures and magnetic fields in underdoped cuprates will also be discussed.