Response evaluation in the assessment of potential new anti-cancer therapies is undergoing intense investigation and change. Current imaging techniques most commonly used in early phase clinical trials are limited to providing reliable and reproducible anatomical data demonstrating a change in size and reduction in tumour volume thereby inferring patient benefit. Current imaging modalities such as computed tomography (CT) and magnetic resonance imaging (MRI) by their nature require computer programs and software. This is a constantly evolving field and upgraded technology enables faster acquisition times for scans, greater anatomical detail and accurate volumetric data to be acquired. Dynamic studies allow contrast agents to be visualised in any given structure over time, so blood flow, blood volume and permeability can be assessed thereby demonstrating function. The advent of many new anti-cancer agents with novel modes of action such as anti-angiogenesis agents act by preventing the development of a suitable blood supply to sustain tumour growth. Such agents do not actively destroy tumour cells so do not exhibit a 'cytocidal' effect as traditional anti-cancer agents do but prevent tumour growth, so can be regarded as 'cytostatic' agents. Therefore, traditional response evaluation criteria may not be appropriate to assess drug efficacy or 'activity' in achieving patient benefit. New techniques have also been developed so the 'function' or metabolism can be demonstrated and tumour serum markers and other factors also require consideration rather than relying on a single modality alone. This article reviews the current accepted response criteria and highlights some newer techniques which will almost certainly play a major role in the assessment of new anti-cancer therapy, particularly in the development of cytostatic agents which are playing an ever increasing role.