The concept of digital roots is over a 1000 years old and is simply the ancient process of ‘casting out 9s’. Apart from digital root properties and applications, the digital roots of polygonal numbers, Fermat numbers, Mersenne primes, perfect numbers, Fibonacci and Lucas numbers, prime and twin primes, amicable numbers, Smith, and Kaprekar numbers are discussed. The digital root concept for fractions, along with recreational topics like additive and multiplicative persistence, is also covered in this chapter.