True Orthodox Churches, also known as "Old Calendarists" or "Catacomb Orthodox Churches," are churches formed in the name of preserving Orthodox Christianity while having limited communication, if any, with the mainstream or canonical Orthodox Church.
Most of these bodies formed in opposition to early forays into ecumenism and in the wake of the Russian Revolution in former territories of the Russian Empire. While their actual numbers are disputed, True Orthodox Churches claim approximately a million adherents.