Within the theoretical framework of knowledge space theory, a probabilistic skill multimap model for assessing learning processes is proposed. The learning process of a student is modeled as a function of the student's knowledge and of an educational intervention on the attainment of specific skills required to solve problems in a knowledge domain. Model parameters are initial probabilities of the skills, effects of learning objects on gaining and losing the skills, and careless error and lucky guess probabilities of the problems. An empirical application shows that the model is effective in assessing knowledge and effectiveness of educational intervention at both classroom and student levels. Practical implications for teaching and learning are discussed.