Develop eloqua customized functionalities

Hello everyone,

I found that applications in the marketplace are developed by third parties (non Oracle developers) which i found interesting because in one hand it offers a variety of amazing functionalities and integrations and in the other hand it shows the ability of creating customized functionalities based on our company's need. My question is : What pre-requisties (technical ones) our consultants and developpers should get in order to start creating Eloqua costumized apps?

Thank you in advance