Over the past two decades research in soil mechanics has shown that the behaviour of clean sands can be described within a Critical State framework similar to that for clays. More recent research has identified that there are many intermediate graded soils for which these types of Critical State framework do not work. These soils have been called ‘transitional’ in that they have a grading and a mode of behaviour between that of clean sands and plastic clays. The main feature of transitional soil behaviour is that unique Normal Compression and Critical State Lines cannot be identified. The compression paths of a transitional soil will show no signs of converging onto a unique line before its maximum density is reached. In this research, oedometer tests, triaxial tests, and particle size analyses were conducted on a range of transitional soils to investigate their compression and shearing behaviour. Existing literature was also searched and analysed for further evidence of transitional soils. This work has shown that transitional behaviour can be seen in a wide range of intermediate graded soils and perhaps in some clays. Examples include silty and clayey sands, residual sandstones, tailings, gap-graded, well-graded and poorly- graded soils, loess and several model intermediate graded soils with various proportions of fine and coarse particles. A consistent set of soil characteristics associated with transitional behaviour could not be established, leading to difficulties in predicting this behaviour directly from the soil characteristics. Triaxial tests on two different transitional soils showed that an infinite number of Critical State Lines can exist for a given soil, one for each initial density, and that many features of the conventional soil behaviour frameworks break down when applied to them.