Most estuaries around the world are dominated by combinations cohesive sediments (generally referred to as mud) and mud:sand mixtures (Manning et al., 2010), the transport and fate of which plays a major role in most estuarine management and many marine engineering projects. Mud typically is composed of mineral grains that originate from both fluvial and marine sources, together with biological matter - both living and in various stages of decomposition (Deng, 2022). It is the combination of these features that makes estuarine mud sticky in nature (e.g. Chassagne et al., 2009; Parsons et al., 2016; Ye et al., 2021), and for this reason these sediment types are referred to generically as cohesive sediments (Whitehouse et al., 2000; Mehta, 2023). In contrast to purely non-cohesive sandy sediments, muddy sediments can flocculate into larger aggregates called flocs (Manning et al., 2017; Spencer et al., 2021, 2022), and this poses a serious complication to modellers of estuarine sediment dynamics. Consequently, understanding the mud processes has been a subject of intensive research effort (e.g. Dyer, 1986; Dronkers and Van Leussen, 1988; Healy et al., 2002) and there has been much progress been made in the past few decades in advancing our understanding of cohesive sediments in aquatic environments, especially estuarial regions. Dyer (1989) proposed key areas that would require research focus on cohesive sediments. It is the aim of this chapter to provide a state-of-the-art review of many of the key scientific advances in research assessing the processes and behaviour of muddy sediments, and the research progress made during the last 20–30-year period. This presentation provides an overview of the following topics: estuarine sediment composition, suspended sediments, flocculation dynamics, cohesion, particle iteraction, mixed sediments, hindered settling, and floc structure. The presentation includes succinct background science and theory on each component topic, together with respective highlights from key research contributions, and also drawing on relevant case studies and real-world examples
