A notion of "monotonic independence" is formulated in the setting of C*-probability space. Based on this independence, a noncommutative central limit theorem and a noncommutative law of small numbers are given.

Let [Formula: see text] be the class of all algebraic probability spaces. A "natural product" is, by definition, a map [Formula: see text] which is required to satisfy all the canonical axioms of Ben Ghorbal and Schürmann for "universal product" except for the commutativity axiom. We show that there exist only five natural products, namely tensor product, free product, Boolean product, monotone product and anti-monotone product. This means that, in a sense, there exist only five universal notions of stochastic independence in noncommutative probability theory.

An example of noncommutative Brownian motion is constructed on the monotone Fock space which is a kind of "Fock space" generated by all the decreasing finite sequences of positive real numbers. The probability distribution at time t > 0 associated to this Brownian motion is shown to be the arcsine law normalized to mean 0 and variance t.

A notion of "quasi-universal product" for algebraic probability spaces is introduced as a generalization of Speicher's "universal product". It is proved that there exist only five quasi-universal products, namely, tensor product, free product, Boolean product, monotone product and anti-monotone product. This result means that, in a sense, there exist only five independences which have nice properties of "associativity" and "(quasi-)universality".

Monotone Lévy processes with additive increments are defined and studied. It is shown that these processes have natural Markov structure and their Markov transition semigroups are characterized using the monotone Lévy-Khintchine formula. 17 Monotone Lévy processes turn out to be related to classical Lévy processes via Attal's "remarkable transformation." A monotone analogue of the family of exponential martingales associated to a classical Lévy process is also defined.