originally thought. Repair systems can remove lesions before they can be converted to mutation, they can also convert initial lesions to secondary ones that are them­ selves mutagenic, and they can remove potentially lethal lesions at the expense of making mutations. The error- avoiding systems asso­ ciated with replication are themselves complex and may be caused to make mistakes in various ways. These different pathways for mutation production and mutation avoidance are still being worked out in prokaryotes and are less well understood in eukaryotes. This symposium shows, however, that very encouraging progress has been made in the last several years, and the progress is now accelerating.