  Given a prime $p$ and cusp forms $f_1$ and $f_2$ on some $\Gamma_1(N)$ that
are eigenforms outside $Np$ and have coefficients in the ring of integers of
some number field $K$, we consider the problem of deciding whether $f_1$ and
$f_2$ have the same eigenvalues mod ${\mathfrak p}^m$ (where ${\mathfrak p}$ is
a fixed prime of $K$ over $p$) for Hecke operators $T_{\ell}$ at all primes
$\ell\nmid Np$.
  When the weights of the forms are equal the problem is easily solved via an
easy generalization of a theorem of Sturm. Thus, the main challenge in the
analysis is the case where the forms have different weights. Here, we prove a
number of necessary and sufficient conditions for the existence of congruences
mod ${\mathfrak p}^m$ in the above sense.
  The prime motivation for this study is the connection to modular mod
${\mathfrak p}^m$ Galois representations, and we also explain this connection.
