This thesis describes experiments on the coherent dynamics and reactions of ultracold, bosonic Cs atoms and Cs$_2$ molecules. Taking advantage of our apparatus, we first apply strong driving to a quasi-two dimensional (2D) BEC by periodically modulating the atomic interaction between repulsive and attractive and observed novel stimulated emission of atomic matter wave jets from condensate into radial directions, which we name as "Bose fireworks". Even though locally the number of atoms in the jets follows thermal distribution, we find the spatial and temporal coherence by interfering two sets of matter wave jets with different momentum and time reversal of the jet emission, respectively. This suggests global unitary evolution of a closed system. By applying dual-frequency interaction modulation schemes, we observed spontaneous formation of density wave patterns with D$_2$, D$_4$ and D$_6$ symmetries emerging from uniform disk-shaped condensates. The patterns are revealed by our unbiased real-space pattern recognition algorithm, which provides richer information than conventional correlation functions. Furthermore, we identified a resonant nonlinear wave mixing process underlying the formation of hexagonal density wave pattern from a novel g$^{3/2}$ correlator.On the other hand, the matter wave emission also serves as a tool for extracting the initial BEC wave function. As two examples, we studied jet emission from a nonuniform BEC with a relative phase between its two halves and a rotating BEC with vortices inside. The relative phase between two halves of the BEC and both the magnitude and chirality of angular momentum of vortices in the rotating BEC are extracted from the substructure of near field matter wave emission patterns. Besides the interaction control, Feshbach resonances also allow us to directly associate atoms in a condensate into molecules. We demonstrated the creation of Cs$_2$ molecular BEC by pairing atoms in an atomic condensate near a narrow g-wave Feshbach resonance, which is confirmed by our equation of state measurement for the molecular quantum gas. We extract the elastic molecular scattering length to be +220 Bohr from the equation of state for the first time. The two-dimensional and flat-bottomed trap geometry and low temperature help to stabilize the molecules and remain thermal equilibrium. Our work thus demonstrates the long-sought transition between atomic and molecular condensates, the bosonic analogue of BCS-BEC crossover in a degenerate Fermi gas. Apart from the equilibrium properties, we also studied reaction dynamics in an atomic BEC by quench magnetic field close to the resonance point. Molecules are produced rapidly from atomic samples after the quench. As a function of the atomic sample temperature, the initial molecule formation rates sharply transition from the values determined by thermal collisions between atoms in normal gas phase to those in the degeneracy regime where the wave nature of atoms dominates. Following the initial proliferation, the molecules reach quasi-equilibrium with the atoms and the number of molecules shows coherent oscillatory evolution with the oscillation frequency determined by both the molecular binding energy and the atomic density. Our experiments thus demonstrate collective chemical reactions in a strongly interacting atomic BEC.