The work is concerned mainly with the slug flow regime in fluidised beds, where bubbles are of a size comparable to the column diameter. The fluidising fluid in all cases was air and the particles used were sand, quartz, ballotini and catalyst. The range of particle sizes was about 30 to 500 microns. Column diameters of 1 to 4 inches were used. Much of the work is concerned with the varying behaviour of slugs in beds of different particles sizes. In Chapter 1 it is shown that slugs tend to travel faster in beds of fine particles where coalescence is completed only at large bed heights. In larger particles, slugs may, at high air velocities, travel faster than suggested by "ideal slug theory", because they tend to travel on the column walls. The spacing between slugs, when coalescence has ceased, is greater in beds of fine particles. In Chapter 2 these differences are considered when predicting the length of slugs at a given air velocity. In Chapter 4 the expansion of slugging fluidised beds is considered, and it is shown that an expansion less than that predicted by "ideal slug theory" can be fully explained by enhanced slug velocities, and is not in any way an indictment of the "two-phase theory" of fluidisation. In Chapter 5 the problems of pressure and bed height fluctuations in slugging beds are considered, and equations formulated to allow for the varying behaviour of slugs in beds of fine and coarse particles. The air velocities within which a bed will remain in the slugging regime are discussed in Chapter 3. A theory, which predicts the onset of slugging , allows for the different slug velocities and spacings to be found in beds of fine and coarse particles . It was found that at air velocities somewhere ln excess of three times the terminal velocity of the finest particles in the bed, the slugging regime broke down to a "turbulent regime" where no discrete slugs existed. This breakdown occurred at air velocities as low as 10 cm/sec. for the finest particles. It is concluded, in Chapter 6, that any "maximum stable" bubble or slug size which exists is intimately connected with this breakdown to the turbulent regime, and will be the result of very rapid coalescence, rather than the entry of particles into a bubble or slug. A conclusion, from Chapters 3 and 6, is that the addition of fine particles to a bed of coarse material can have a marked effect on the fluidising behaviour of the bed. In Chapter 7 the rate at which these fine particles will be carried out of a bed of coarse material is considered, and is related to the slugging behaviour of the bed. Finally, in Chapter 8, such experimental details, as have been incompletely considered in the preceeding chapters, are discussed.