Jacobi theta functions form a class of complex functions showing remarkable features. Due to such properties, Jacobi theta functions appear in different, completely unrelated, disciplines in mathematics and physics. In a natural manner, Jacobi theta functions give rise to elliptic functions, which I discussed a year ago during this seminar. My presentation will be a continuation of that seminar. I will briefly survey the definition and basic properties of the Jacobi theta function and I will show its relations to differential equations and the theory of modular functions.