• Subject ≥ 18 years of age

• Subjects with histologically proven high risk stage II or stage III colon or rectal cancer treated with curative intent with surgery alone (any T, N1 or N2) with no evidence of metastatic disease. High risk stage II is defined as having one or more of the following: T4 disease, obstruction and/or perforation of the primary tumour during the pre-operative period, inadequate nodal harvest as indicated by \<12 nodes examined, poorly differentiated grade on histology, perineural invasion, peritoneal involvement or extramural venous/lymphatic invasion. Subjects must be due to receive adjuvant chemotherapy after surgery or Subjects with histologically proven locally advanced stage III rectal cancer treated with neoadjuvant chemoradiotherapy (any T, N1 or N2, M0) with no evidence of metastatic disease are eligible. Subjects must be due to receive adjuvant chemotherapy after surgery

• Fully surgically resected tumour with clear resection margins (i.e., \>1 mm).

• Adequate organ function

‣ Absolute neutrophil function ≥1.0 x 109/ L

⁃ Platelet Count ≥ 75 x 109 / L

⁃ Haemoglobin ≥80g/L (blood transfusion before randomisation is allowed)

⁃ Adequate renal function (GFR ≥ 50ml/min if single agent capecitabine or CAPOX being administered) as calculated by Cockcroft and Gault equation

⁃ Aspartate aminotransferase/ Alanine aminotransferase levels ≤ 2.5 upper limit of normal

• Absence of major post-operative complications or other clinical conditions that, in the opinion of the investigator, would contraindicate adjuvant chemotherapy

• Patients should be assessed by Oncology team for suitability and assessment for adjuvant chemotherapy, be able to have post-operative ctDNA sample collected and be randomised by week 8 ± 2 weeks after surgery.

• ECOG performance status 0- 2

• Able to give informed consent