The technique of error-trapping decoding for algebraic codes is studied in combinatorial terms of covering systems. Let

, and

be positive integers such that

. An

-covering system is a pair

, where

is a set of size

and

is a collection of subsets of

, each of size

, such that for all

of size

, there exists at least one

with

. Let

denote the smallest size of

, such that

is an

-covering system. It is shown that the complexity of an error-trapping decoding technique is bounded by

from below. Two new methods for constructing small

-covering systems, the algorithmic method and the difference family method, are given.