All access to the APSIM software is through approved licence agreements.
Licensed users may access and modify all source code. All modifications to APSIM must be submitted to the AI and be subject to the Reference Panel evaluation process before inclusion into the official APSIM release. Instructions for Submitting Improvements to APSIM for Review by the APSIM Initiative Reference Panel can be found here.
It can be useful to notify the APSIM Initiative that you are planning on making an improvement to APSIM. This can often lead to others suggesting ideas or collaborators to work with. To do this you need to create an issue in GitHub with a description of what you intend doing. This is for APSIM 7.x and APSIM Next Generation.
You will need to agree to one of two types of licences before downloading the software: