This article introduces the idea that probabilistic reasoning (PR) may be understood as information compression by multiple alignment, unification and search (ICMAUS). In this context, multiple alignment has a meaning which is similar to but distinct from its meaning in bio-informatics, while unification means a simple merging of matching patterns, a meaning which is related to but simpler than the meaning of that term in logic. A software model, SP61, has been developed for the discovery and formation of "good" multiple alignments, evaluated in terms of information compression. The model is described in outline. Using examples from the SP61 model, this article describes in outline how the ICMAUS framework can model various kinds of PR including: PR in best-match pattern recognition and information retrieval, one-step "deductive" and "abductive" PR, inheritance of attributes in a class hierarchy, chains of reasoning (probabilistic decision networks and decision trees, and PR with "rules" ), geometric analogy problems, nonmonotonic reasoning and reasoning with default values, modelling the function of a Bayesian network.