The following lecture notes offer a short, elementary introduction to the field of high-intensity laser interactions with matter. The material starts with basic phenomena such as ionization, plasma characterization and thresholds for non-linear behaviour. In the latter sections, topics relevant to laser-based particle acceleration are covered in detail, including wave propagation in underdense plasmas and fast electron heating in overdense (solid) plasmas. The notes are concluded with a short summary of plasma simulation techniques, including a work-through tutorial using a particle-in-cell code.