Abstract:  Classical theta series are among the simplest and most useful of modular forms, with applications ranging from cryptography and sphere packing to differential geometry. In this talk, which will be informal and will not assume any previous knowledge, I will give several examples of how to work with theta series, including a very short proof of the "Watson quintuple product identity", a surprising result about the vanishing of certain special values of theta functions, and examples of the theory of "theta blocks", including the so-called "theta quarks".