Microbial granules play an important role in the field of biological wastewater treatment due to their advantages over the conventional sludge flocs, such as a denser and stronger aggregate structure, better settleability and ensured solid-effluent separation, higher biomass concentration, and greater ability to withstand shock loadings. A better understanding of microbial granules may help in engineering biological wastewater treatment systems. Recent studies have greatly expanded our vision over the physicochemical characteristics of microbial granules. This paper provides an up-to-date review on recent work in the understanding of physicochemical characteristics of both anaerobic and aerobic granules with regard to settleability, permeability, morphology, mechanical stability, rheology, porosity, surface adsorbability, surface hydrophobicity and thermodynamics, and extracellular polymeric substances. Our growing knowledge on such characteristics might facilitate the engineering and optimization of microbial granulation as one of the most promising techniques in biological wastewater treatment.