Foamed concrete has proven to be an effective alternative to granular fills and is now widely used internationally. With increasing demand for lightweight materials for buildings in order to improve sustainability, foamed concrete has also developed as an ideal material for this purpose, and many countries utilise construction with precast foamed concrete blocks. However, at densities lower than current technology allows, typically <500 kg/m3, foamed concretes are more prone to instability of the fresh mix. Furthermore at very low densities, ≤300 kg/m3, instability is almost inevitable, greatly limiting the potential of foamed concrete for applications where mass is critical (e.g. weak soils, backfilling damaged structure etc.). This paper aims to illustrate the mechanisms of stability and instability in foamed concretes and demonstrates how ultra-low density mixes (down to plastic density of 150 kg/m3) can be successfully produced.