The contribution of the trunk muscles to spinal stability is well established. There is convincing evidence for the role of multifidus in spinal stability. Recently, emphasis has shifted to the deep fibres of this muscle (DM) and five key clinical beliefs have arisen: (i) that DM stabilizes the lumbar spine whereas the superficial fibres of lumbar multifidus (SM) and the erector spinae (ES) extend and/or rotate the lumbar spine, (ii) that DM has a greater percentage of type I (slow twitch) muscle fibres than SM and ES, (iii) that DM is tonically active during movements of the trunk and gait, whereas SM and ES are phasically active, (iv) that DM and the transversus abdominis (TrA) co-contract during function, and (v) that changes in the lumbar paraspinal muscles associated with LBP affect DM more than SM or ES. This paper reviews the biomechanical, electromyographic, histochemical and morphological data that underpin these beliefs. Although there is support for the importance of the lumbar multifidus and the specific contribution of this muscle to intervertebral control, several of the clinical beliefs have little or no support and require further evaluation. These findings have implications for clinical practice.