Life is hierarchally structured, with replicating entities nested within higher order self-replicating structures.  Take, for example, multicellular life: the multicellualr entity replicates, as do the cells that comprise the origanism.  Inside cells are mitochondria that also have capacity for autonomous repliaction; the same is true of chromosomes within the nucleus, and of genes that comprise chromosomes.  Such hierarchal structure reflects a series of major evolutonary transitions in which lower order self-repliacing entities have been subsumed within higher order structures.  Typically this involves the lower level entity "giving up its right to autonomous replication" and with this "sacrifice" comes enslavement to the "needs" of the higher order "corporate body".  Posed in these terms it is difficult to see how evolutionary transitions unfold; how selection might shift levels and why life is hierarchally structured.  Necesssary for progress is clarity concerning what needs to be explained.  I will argue that this is the evolution of Darwinian individuality -- the evolution of properties of entities (variation, reproduction and heredity) that ensure participation in the process of evolution by natural selection.  There has been a tendency to assume these properties as pre-existing but they are not: they are derived and require evolutionary explanation.  Pressing to the heart of the problem, the challenge is to explain how Darwinian properties emerge from non-Darwinian entities by non-Darwinian means.  THis challenge permeates each evolutionary transition including the emergence of life from non-life.  I will argue that solutions to this seemingly unsolvable problem arise once we consider ecology.  I will also argue that the feedback between organismal charcteristics and the amplifying effects of natural selection makes evolutionary transitions in indivduality inevitbale, and thus too, life's hierarchal structure.  Theoretical ideas will be augmented with experimental data on the evolution of multicellular life.