Cache memories are now standard components of modern
computer systems. They have proven extremely useful in bridging the
gap between CPU and DRAM speeds, which continues to
grow. Consequently, there has been a great deal of research into
making caches more aggressive. A specific type of cache is the ``trace
cache'' which stores dynamic sequences of instructions as opposed to
sequential contiguous blocks. It is the aim of this paper to provide
an overview of trace caches against a background of enhancements to
standard caches. The basic trace cache architecture is
presented together with a survey of several enhancements which 
have been made to improve its performance. Following this the
limitations of trace caches are discussed.