Transition-metal dichalcogenides (TMDs) exhibit a phase diagram in which a multitude of charge density wave (CDW) orders can coexist and/or compete with superconductivity. Despite extensive studies over the last decade, the mechanisms of CDWs and superconductivity in TMDs are still under debate. In the first part of the colloquium, I will provide a review of CDW in MDs. I will then discuss our recent observations of dynamically modulated quantum phase transition between two distinct charge density wave (CDW) phases in two-dimensional 2H-NbSe2. We find a surprising evolution of the conductance fluctuation spectra across the CDW temperature: the conductance fluctuates between two precise values,separated by a quantum of conductance. With the help of mean-field calculations, these observations could be explained as to arise from dynamical phase transition between the two CDW states. The results resolve a long-standing mystery of the anomalously large spectroscopic gap in NbSe2. I will also discuss our recent results on observation of signatures of pseudogap state at temperatures above TCDW in two-dimensional 2H-NbSe2. These results might explain the difference in the CDW transition temperature inferred from transport and Raman spectroscopy measurements.