  Quantifying the difference between two probability density functions, $p$ and
$q$, using available data, is a fundamental problem in Statistics and Machine
Learning. A usual approach for addressing this problem is the likelihood-ratio
estimation (LRE) between $p$ and $q$, which -- to our best knowledge -- has
been investigated mainly for the offline case. This paper contributes by
introducing a new framework for online non-parametric LRE (OLRE) for the
setting where pairs of iid observations $(x_t \sim p, x'_t \sim q)$ are
observed over time. The non-parametric nature of our approach has the advantage
of being agnostic to the forms of $p$ and $q$. Moreover, we capitalize on the
recent advances in Kernel Methods and functional minimization to develop an
estimator that can be efficiently updated online. We provide theoretical
guarantees for the performance of the OLRE method along with empirical
validation in synthetic experiments.
