The forum, sponsored by the Orange Chatham Group of the Sierra Club, is scheduled for 7 p.m. and will give group members and residents an opportunity to interact with candidates.
Dan Coleman, the club's political chairman, said some issues that will be discussed could include traffic, greenways, water and air qualities, growth and development and possible University expansion.
"(The forum will) give our members and the general public the chance to learn the candidate's views on important environmental issues," Coleman said.
Gustavo Ocoro, vice chairman of the Sierra Club, said the club will consider endorsing a specific candidate after tonight's forum.
"If we have a candidate that goes with our philosophy that we think we should support, then we would do that," he said.
All three of the mayoral candidates are expected to attend the event.
Kevin Foy, a Town Council member and the one of the candidates, said he expects the meeting to focus on growth and development.
But Foy said it does not matter whether a candidate is pro-development or not.
"It's a matter of managing (growth) responsibly," Foy said.