Laughter and smiles are one of the most common and marked expressions of happiness in humans. They bear resemblance in form and function to great ape vocal and facial displays that occur during tickling sessions and social play. The three main studies of this book, which were conducted by Marina Davila Ross for her PhD, apply a comparative approach across great apes and humans to assess the function, test for mimicry, and reconstruct the evolutionary relationships of these displays.