Floc structure is an essential factor in determining the treatment performance of sludge processes. This work briefly reviews the feasibility of some commonly adopted microscopic and tomographic techniques for investigating the sludge floc structure. A wastewater sludge was investigated using the following techniques: confocal laser scanning microscopy, sanning electron microscopy, praffin-embedded microtome slicing, small-angle light scattering, free settling, atomic force microscopy, transmission electron microscopy, magnetic resonance imaging, computerized auxiliary tomographic scanning and electrical capacitance tomography. The outcomes of these tests were reported and a comparison was made based on their capability to probe the sludge at different scales.