A new concept, namely the error discrimination of a code defined as the capability to not only detect errors from two distinct error sets but also to distinguish between them has been introduced in [SAKA 89a]. Consider $E_+;and;E_-$ as the two distinct error sets, namely the positive error set and the negative error set respectively. If a code C is not only capable of detecting any error e in {$E_+,;E_-$}, but also able to identify the error set to which e belongs then the code is said to be an $E_+;&;E_-$ error discriminating code. The error discriminating property enables construction of unidirectional error detecting/correcting codes using asymmetric error control code. We derive here theory for asymmetric t error correcting and d error detecting codes. Furthermore, unidirectional error control code construction methods are introduced using asymmetric error control codes and $E_+;&;E_-$ error discriminating codes.