This monograph consists of three chapters covering the following topics: Foundations, Bilinear forms and presentations of certain 2-step nilpotent Lie groups, Discrete subgroups of the Heisenberg group, The automorphism group of the Heisenberg group, Fundamental unitary representations of the Heisenberg group, The Fourier transform and the Weil-Brezin map, Distinguished subspaces and left action: Jacobi theta functions and the finite Fourier transform, Nil-theta functions and Jacobi-theta functions, and the algebra of the finite Fourier transform; Abelian varieties, nil-theta and theta functions.It includes: a general construction and algebraic foundations, Nil-theta functions associated with a positive definite H-morphism of an Abelian variety, and the relation between nil-theta and classical theta functions. In presenting the material, the author has attempted to lay a careful foundation and has stressed low-dimensional examples and special computations when proving general results by general techniques. This an expository piece of work, although some of the results are new.