The Great Migration is the largest animal migration in the world. Each year, more than two million animals (wildebeest, zebra, and gazelle) migrate in a clockwise direction across the ecosystems of the Serengeti (Tanzania) and the Masai Mara (Kenya). While this cycle continues throughout the year, August and September are the best times to see the Great Migration in Kenya and your chances of witnessing a river crossing are generally much higher. Come early October, the rainy season in Tanzania will commence, and the herds will embark on their return journey back to the Serengeti.