Green hydrogen energy driven by water electrolyzers and fuel cells presents a promising future. The fluid (gas/liquid) transport in practical water electrolyzers and fuel cells pose one of the most critical challenges limiting their efficiencies. This dissertation investigates ultrasonic measurement techniques towards resolving fluid transport on the different heterogenous scales in real-time, ranging from several millimetres in the porous electrodes to 100’s mm in the flow field, providing high spatial resolutions of 10’s µm to 100’s µm, respectively. First, volumetric scanning acoustic microscopy imaging for quantifying fluid transport in the porous electrodes of water electrolyzers was investigated. High spatial resolutions of 10’s µm were achieved, allowing for visualizing the electrolysis produced bubbles in the electrodes at the pore scale of 100’s µm, and for quantifying their volumes, which are directly related to the efficiency loss. Towards real-time and long-term monitoring of large electrolyzers, a compressive imaging using a single element transducer scheme was investigated. With the single snapshot acquisition scheme, the compressive imaging drastically increases the temporal resolution by several orders of magnitude over the SAM imaging, expanding the borders of imaging volumes. Ultrasonic guided waves (UGWs) echo localization based techniques were investigated for monitoring the water droplets and gas bubbles in the entire flow fields of large scale electrolyzers and fuel cells. The UGWs propagating in the flow channels are scattered by any presences of water droplets or gas bubbles in the flow channels. Droplets as small as 50 nL and bubbles with diameters of 150 µm were accurately localized with uncertainties less than 500 µm.