The origin of multicellularity was one of the most significant innovations in the history of life. Our understanding of the evolutionary processes underlying this transition remains limited, however, mainly because extant multicellular lineages are ancient and most transitional forms have been lost to extinction. We bridge this knowledge gap by evolving novel multicellularity in the lab, using the 'snowflake yeast' model system. In this talk, I'll focus on our ongoing Multicellular Long-Term Evolution Experiment (MuLTEE), in which we've put snowflake yeast through ~8,000 generations of selection for larger size and faster growth. We will examine key steps in the evolution of multicellularity, namely how multicellular traits arise and become heritable, how simple multicellular bodies evolve to become radically stronger and tougher, how cells divide labor through differentiation, and how groups overcome diffusion limitation by generating rapid hydrodynamic flows. Overall, our approach allows us to examine how simple groups of cells can evolve to become increasingly integrated and organismal, providing novel insight into this major evolutionary transition.