The conception of development as a movement from a non-differentiated whole to a differentiated and structured system corresponds to a lot of facts. But now the general notion of development can be specified and corrected in the light of new approaches in the frameworks of the theory of self-organizing systems (synergetics), the complexity theory and the idea of global evolutionism. In this context new problems arise, which are being heatedly discussed. One of these is the problem of complexity. It is possible to speak about the measures of complexity and the increase in complexity in the process of development. Complexity can correlate with differentiation. But it seems that the notion of complexity is more exact than that of differentiation, as the former can be measured. In the process of development there is not only a transition from a primary whole to differentiation (it usually happens in the framework of a single system), but also arising of new systems and new system levels. Relations between old and new systems can be various. New systems can supplant old ones. A new system level can arise on the basis of a primary system – as if its superstructure. Properties and processes of a higher level cannot be understood on the basis of knowledge of properties and processes of a primary level. Thus there is a case usually termed "emergence", i. e. the new cannot be reduced to processes at a base level. It is important that the higher level transforms the basic one and determines its functioning. Certainly, in the process of development a transition to a greater differentiation (greater measure of complexity) happens. But it is not an indivisible process: new system and new systemic levels have a differentiation which is not identical to a previous one, often a new differentiation cannot be considered as a simple continuation of the latter one.