The development of Indonesian contract law has been influenced by enacment of new law, court verdicts and legal practices. It has been influenced by civil law and common law systems applied in other countries as well. The enacment of Consumer Protection Act strenghtens position of consumers against profesional seller. The Basic Agrarian Law and its Implementation Laws improve certainty in ownership of land. Courts have recognized, the doctrine of undue influence, acceptance by conduct, but they have not recognized pre contractual liability and have not applied the doctrine of unjustified enrichment in disputes regarding illegal contract. As practical matter, the integration clause under common law system stipulated in contract governed by Indonesian law, whereas Internasional Convention on Sale of Good regarding this issue adopts civil law system.