Vector-space theoretic analysis that has been used to deal with the cycle structure of (finite) linear and complemented cellular automata in Chap.  7 is not applicable to non-linear cellular automata (CAs). A different method is needed to explore the configuration space of a non-linear cellular automaton (CA) and to find cycles in it. Reachability tree, a tool to characterize non-linear CA (see Sect.  3.3 ), is explored in this chapter for the purpose. The chapter concentrates only on the non-uniform finite CAs with ECA rules.