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Foamed concrete has considerable potential as a material for use in the construction industry. However, although some researches have been conducted on the characteristics of foamed concrete, thus far, knowledge on the behaviour of foamed concrete has been limited. Hence, predictions of the stability of foamed concrete under different conditions and mix constituents have been uncertain. The aim of the presented study is to investigate causes of instability of foamed concrete by examining its rheological properties and microstructure. This study explores the complex causes of instability in foamed concrete by examining the rheological parameters, the yield stress and the plastic viscosity, since the rheological properties affect the hardened state. Using flowability as a guide, the relationships are examined between yield stress and viscosity, specifically with reference to their effect on density and w/c ratio. Other factors affecting the rheological properties related to the proportions and fineness of the mix constituents are also considered. Thereafter, the microstructure of foamed concrete is examined to establish links with the rheological values and the relationship with stability. The microstructure, best described by the bubble sizes, has been found to be a function of yield stress, plastic viscosity, material fineness and surfactant types. The bubble diameters have been shown to range between 0.1 to 0.5 mm. Bubbles less than 0.35 mm in diameter correspond to stable mix with a drop in level of less than 5% in height in densities of 1000 kg/m3 and higher. The big bubbles link to unstable mixes and have been found to be a source of instability. Other chemical additions were shown to result in disintegration of bubbles. As this study unfolds, a relationship is established between bubbles and the yield stress values. Bubble sizes reduced when the yield stress increased. For flowability out of Marsh cone test taken between 1 to 2 minutes, the corresponding yield stress was between 6.0 N/m2 to 8.5 N/m2. For this range, the empirical bubble sizes were found to be between 0.33 to 0.35 mm in diameter. In examining the possible causes of instability, it was found that stability improved markedly with increase in density and lesser effect by other factors such as w/c ratio, constituent materials and specimen height. However, the rate of hardening was a dominant factor in stability as evidenced by the use of Calcium Sulfoaluminate cement, CSA and CEM I 52.5R cement which increased the setting times. Stability was drastically achieved even at lower density 300 kg/m3. Blends of CSA with CEM I 52.5R and fine fly ash, FAf, demonstrated similar results. This research has implications for the development of foamed concrete as a material that could be more widely used in certain construction contexts where stability in lightweight density foamed concrete is crucial. It has contributed to better understanding of the rheological properties and the effect on the microstructure, even though the results are based on empirical values. Hence, it is anticipated that the prediction of stability can be made through a selection of materials and proportions to suit different contexts and their requirements.