Lattice paths and submonoids of Z^2

We study a number of combinatorial and algebraic structures arising from walks on the two-dimensional integer lattice. To a given step set $X\sub\Z^2$, there are two naturally associated monoids: $\F_X$, the monoid of all $X$-walks/paths; and $\A_X$, the monoid of all endpoints of $X$-walks starting from the origin $O$. For each~${A\in\A_X}$, write $\pi_X(A)$ for the number of $X$-walks from $O$ to $A$. Calculating the numbers~$\pi_X(A)$ is a classical problem, leading to Fibonacci, Catalan, Motzkin, Delannoy and Schr\"oder numbers, among many other famous sequences and arrays. Our main results give the precise relationships between finiteness properties of the numbers $\pi_X(A)$, geometrical properties of the step set~$X$, algebraic properties of the monoid~$\A_X$, and combinatorial properties of a certain bi-labelled digraph naturally associated to $X$. There is an intriguing divergence between the cases of finite and infinite step sets, and some constructions rely on highly non-trivial properties of real numbers. We also consider the case of walks constrained to stay within a given region of the plane, and present a number of algorithms for computing the combinatorial data associated to finite step sets. Several examples are considered throughout to highlight the sometimes-subtle nature of the theoretical results.