Inflation is a period of accelerated expansion in the very early Universe that is typically invoked as a solution to the problem of originating the observed cosmic microwave background anisotropies, as well as those of the original hot big bang model. The particle content of the inflationary era is as-of-yet unknown, hence it is imperative that the best models of inflation are studied carefully for their potentially unique observational characteristics and then compared to current observations in a statistically rigorous way. In this thesis we will primarily demonstrate how additional scalar degrees of freedom - which are motivated from many high-energy embeddings - open up new observational windows onto the physics of inflation. We construct a Bayesian framework to statistically compare models with additional fields given the current astronomical data. Putting inflation to the test, we perform our analysis on the quadratic curvaton accompanying a range of inflationary potentials, where we find that only one potential remains as a viable candidate. Furthermore, if the curvaton mechanism were to be confirmed by future non-Gaussianity measurements (from large scale structure surveys), the model could prove to be tremendously informative of the early inflationary history. The initial conditions given to these scalar fields become apparent when considering their fundamentally quantum behaviour. Taking this physics into account leads us to develop detailed models for post-inflationary phenomenology (namely, the curvaton and freeze-in dark matter models) and to discover powerful new probes of inflation itself. We further demonstrate how this theoretical study complements our statistical approach by motivating the prior information in our Bayesian analyses. The thesis finishes with a discussion of the future prospects for inflationary model selection. By hypothesising different toy survey configurations, we forecast different outcomes using information theory and our newly developed Bayesian experimental design formalism. In particular, we find that the most likely observable to optimise model selection between single-field inflationary models, through an order of magnitude precision improvement in the future, is the scalar spectral index. We conclude with a summary of the results obtained throughout.