The optical measurement of tumors in the breast typically requires the measurement of multiply scattered near-infrared (NIR) light, which has traveled up to several centimeters in depth. Due to the diffusive nature of photon propagation in thick biological tissue, techniques that employ these measurements are usually referred to as diffuse optical imaging (DOI) or diffuse optical spectroscopy (DOS).1 These methods have been widely shown to be capable of extracting tissue level concentrations and/or changes in concentration of key functional parameters in the breast, brain, muscle, and other tissues. Depending on wavelengths utilized, the parameters may include oxyhemoglobin, deoxyhemoglobin, water, and lipids.2 Diffuse optical techniques can be broadly classified into three variants: continuous-wave (CW), frequency-domain (FD), and time-domain (TD) methods. FD and TD techniques provide separation of optical absorption and scattering effects but typically require modulated or pulsed laser sources and sensitive avalanche photodiodes or photomultiplier tube detectors.3–5 CW techniques can provide relative changes in tissue chromophores if assumptions or prior knowledge of the wavelength dependence of scattering are available.6 In situations in which the optical scattering can be assumed to be time-invariant, CW DOI or DOS can be used to monitor hemodynamic and metabolic changes in tissue with relatively simple instrumentation and analysis techniques. This is especially relevant over short time periods (seconds to hours).7–9 Measurements are possible with inexpensive LEDs and photodiodes, and surface-mount packaging of these components reduces the device footprint and facilitates the design of flexible optical probes that can be used in direct contact with skin. These features provide the possibility for wearable CW imaging probes for measurements of deep tissue hemodynamics. Several optical wearables have been previously developed and applied for studies related to cardiology,10 general purpose blood oxygen saturation,10,11 and cerebral activity,11–15 but we know of no prior work in optical wearables for tumor chemotherapy monitoring.