We usually expect that if a Diophantine equation has non- trivial solutions in the reals and the p-adic numbers for every prime p, then it should have a non-trivial rational solution. If this holds for a given equation, then we say that it satisfies the Hasse principle. In this talk we will discuss an analogue of the Hasse principle for describing when an equation has solutions in the prime numbers. We also duscuss how this can be shown to hold for almost all homogeneous equations with enough variables using the geometry of numbers.