The classification of cacti has been an ever-changing task that has consisted of a good deal of naming and renaming. This is due in large part to the extremely variable growth habits of most cacti. Many cacti plants of the same species may appear quite different from one another. Cacti also reproduce proficiently through vegetative means creating large numbers of a particular variation and therefore furthering the impression that what is actually a variety is instead a unique species.

Because of this, early publications written by both amateurs and professionals that described and classified cacti created a lot of confusion as to which names and groups were to be accepted and which were not. Most classification was performed by individuals up until 1984 when the International Cactaceae Systematics Group was formed to standardize the classification of cacti.

Despite the efforts of the International Cactaceae Systematics Group (ICSG), people today are still faced with inaccurate and/or outdated information through many on-line and printed resources as well as through word of mouth. This creates confusion for those attempting to identify an individual cactus plant.

In classifying the species listed on Cactiguide.com, I have decided to use Edward F. Anderson's book, The Cactus Family, whenever possible. In this book he states,

"I would not pretend to work independently of the many specialists and colleagues with whom I have been associated in this project since 1984. Rather, I defer to the judgment of the International Cactaceae Systematics Group and the classification system used in this book is essentially that of the experts who make up the group."

Rather than my trying to duplicate his work, I have also used Anderson's The Cactus Family to list the appropriate synonyms of each species. I have not, however, listed the name of the person who first described the plant as is the custom to do because it is not necessary for identification.

Because many names that were rejected by the ICSG are still heavily used today, I have listed the many synonyms for each species in accordance with those listed in The Cactus Family.

NOTE: CactiGuide.com is intended as a source for the positive identification of a particular cactus plant. For further study including biological information, histories, conservation, and growing tips, I strongly recommend other sources such as The Cactus Family, which covers all of those subjects in detail.

-Daiv Freeman