The influence of a long stay in Rome is seen in his treatment of landscape and backgrounds. One of his important contributions is the introduction to Roman painting of new subjects derived from Flemish and Dutch genre paintings including according to a contemporary source, "rogues, cheats, pickpockets, bands of drunks and gluttons, scabby tobacconists, barbers, and other 'sordid' subjects." His subjects also included blacksmiths shoeing horses in grottoes, travelers in front of inns, brigands attacking travelers, military actions, idlers around Roman lime-kilns, markets, feasts and scenes with hunters. He further depicted people playing popular games of chance such as morra as well as excretory functions. Several of his compositions deal with lively scenes from peasant life. His pictures are marked by skillful composition and good drawing. He was especially careful in perspective. Despite their lowly subject matter, van Laer's works themselves sold for high prices and were held in some of the most prestigious collections of his time.