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The main purpose of this paper is to study time-inhomogeneous one-dimensional branching processes (mainly on a continuous but also on a discrete state space) with the help of recent achievements in Loewner Theory dealing with evolution families of holomorphic self-maps in simply connected domains of the complex plane. Under a suitable stochastic continuity condition, we show that the families of the Laplace exponents of branching processes on $[0,\infty]$ can be characterized as topological (i.e. depending continuously on the time parameters) reverse evolution families whose elements are Bernstein functions. For the case of a stronger regularity w.r.t. time, we establish a Loewner - Kufarev type ODE for the Laplace exponents and characterize branching processes with finite mean in terms of the vector field driving this ODE. Similar results are obtained for families of probability generating functions for branching processes on the discrete state space $\{0,1,2,\ldots\}\cup\{\infty\}$. In addition, we find necessary and sufficient conditions for "spatial" embeddability of such branching processes into branching processes on $[0,\infty]$. Finally, we give some probabilistic interpretations of the Denjoy - Wolff point at $0$ and at $\infty$.