We present results for quark cluster probabilities in nuclei based on the assumptions of the quark cluster model. We first perform numerical evaluations in a= 2, 3, and 4 nuclei based on realistic nuclear wave functions for the probability that a quark chosen at random in the nucleus is found in a color singlet cluster consisting of 3, 6, 9, etc., quarks. Clustering itself is determined by a geometrical overlap of three-quark systems that depends on a critical distance of separation 2 R c. A systematic comparison of cluster probabilities obtained for these light nuclei establishes certain features of these results, which are independent of A and of the wave function used. These results are then used to make predictions of quark cluster probabilities in heavier nuclei. We present relationships between our definition of quark cluster probabilities and that of some other efforts.