Escape rate measurements of the critical current in YBaCuO grain boundary Josephson junctions demonstrate the existence of Macroscopic Quantum Tunneling (MQT) of the superconductive phase in a d-wave high temperature superconductor (HTS). The Josephson current-phase relation is significantly modified in a junction with tunneling between a lobe and a node in the gap functions on either side of a 45 degree grain boundary. An additional sin2f component has to be considered. Dissipation can be partly controlled and suppressed in specific junction configurations and as high a Q-value as 40 was measured. Microwave measurements show the existence of quantized excitation energy levels at low temperature, confirming low dissipation. The results show quantum coherence in HTS junctions and may be of importance for possible applications of d-wave superconductors in quantum computing.