Measurements of single top-quark production in proton--proton collisions are presented based on the 8 TeV and 13 TeV ATLAS datasets. In the leading-order process, a $W$ boson is exchanged in the $t$-channel. The fiducial cross-sections measurements for the production of single top-quarks and single anti-top-quarks, as well as the total cross-sections and their ratios are presented. At 8 TeV, differential cross-section measurements of the $t$-channel process are also reported, these analyses include limits on anomalous contributions to the $Wtb$ vertex and measurement of the top quark polarization. A measurement of the production cross-section of a single top quark in association with a $W$ boson, the second largest single-top production mode, is also presented. Finally, evidence for $s$-channel single-top production in the 8 TeV ATLAS dataset is presented. All measurements are compared to state-of-the-art theoretical calculations.