Cancer research revolves around the relationship between the immune system and the tumour microenvironment (TME), and regulatory immune cells have emerged as a vital factor influencing this complex relationship. This chapter focuses on regulatory T cells (Tregs), regulatory B cells (Bregs), myeloid-derived suppressor cells (MDSCs), and other regulatory immune cells that contribute to immune homeostasis in the TME, resulting in pro-tumour and anti-tumour effects through a variety of methods. In addition to inhibiting effector cells, disrupting antigen presentation, and altering checkpoint pathways and metabolic landscapes, these cells provide an immunosuppressive microenvironment that promotes tumour growth and progression. Current research trends include analysing single-cell transcription, investigating metabolic control mechanisms, and developing combination therapies targeting both effector-regulatory immune responses. By unravelling the complex network of regulatory cells in cancer, researchers aim to create novel therapeutic strategies tailored to individual immune profiles and drive clinical outcomes.