The liquid-film thickness behavior is a key parameter on the heat transfer process during flow boiling and convective condensation, affecting the heat transfer coefficient and dryout incipience. A deep knowledge regarding its transient characteristics is relevant for the development of phenomenological models for the dryout and heat transfer coefficient in annular flows. In this context, the present lecture describes the results of liquid-film thickness measurements performed during flow boiling and convective condensation of R600a, R290, R1270, and the mixtures R600a/R290 and R600a/R1270 in a horizontal tube (9.43 mm internal diameter) for vapor qualities ranging from 0 to 1. The reported experimental data comprises results for the instantaneous liquid-film thickness, interfacial wave velocity (ripples and disturbance waves) and intermittent dryout-rewetting phenomenon. The liquid-film thickness measurements were obtained simultaneously to local heat transfer coefficient measurements. High speed images of the flow were also gathered for the same experimental conditions. Care was exercised in the discussions presented in this lecture to establish a strong link between liquid film characteristics and the associated heat transfer behaviors.