We have studied the phase behaviour of both charged and uncharged hard colloidal particles in a solution of wormlike micelles of the nonionic surfactant C₁₆E₆. The behaviour found is similar to that in the case of colloid-polymer mixtures. The phase boundaries agree well with theoretical predictions of an adapted colloid-polymer depletion model that accounts for the concentration induced growth and the exponential size distribution of the micelles. In the case of uncharged colloids a stable one phase fluid region is found at low surfactant concentrations. As the concentration of C₁₆E₆ increases, phase separation into coexisting colloid-rich and colloid-poor phases is observed, as a result of the increase of the concentration and/or average length of the micelles. At higher concentrations a metastable gel is found. Similar behaviour is found in the case of charged colloid, both with and without added electrolyte to screen the coulombic repulsion between the particles. A more realistic, industrial type system with charged colloids in an aqueous solution of anionic micelles of sodium lauryl ether sulfate (SLES) and cocoamidopropylbetaine (CAPB) was also investigated. A ternary phase diagram of SLES/H₂O/NaCl, where the salt is used to promote the formation of wormlike micelles, resembles that found in the C₁₆E₆ systems and agrees qualitatively with the theoretical predictions. At low surfactant/salt concentrations the mixture is a one phase fluid. As the concentration of surfactant and/or salt is increased first phase separation into coexisting colloid-rich and colloid-poor phases, then colloidal gels are observed. The collapse of these gels was studied by diffusive-wave spectroscopy and shows a qualitatively reproducible behaviour of the particle dynamics during the gel collapse.