RP1 is a genetically modified herpes simplex type 1 virus that is designed to directly destroy tumors and to generate an anti-tumor immune response. This is a Phase 1/2, open label, multicenter, dose escalation and expansion, first-in-human (FIH) clinical study to evaluate the safety and tolerability, biodistribution, shedding, and preliminary efficacy of RP1 alone and in combination with nivolumab in adult subjects with advanced and/or refractory solid tumors. The study will include a dose escalation phase for single agent RP1, an expansion phase with a combination of RP1 and nivolumab and a Phase 2 portion in specified tumor types for the combination therapy.