The present invention generally relates, in certain aspects, to relatively small devices applied to the skin, modular systems, and methods of use thereof. In some aspects, the device is constructed and arranged to have more than one module. For instance, the device may have a module for delivering to and/or withdrawing fluid from the skin and/or beneath the skin of a subject and a module for transmitting a signal indicative of the fluid delivered to and/or withdrawn from the skin and/or beneath the skin of the subject, a module for analyzing a fluid withdrawn from the skin and/or beneath the skin of the subject, or the like. In some embodiments, the modules are connectable and/or detachable from each other, and in some cases, the connections and/or detachments may be performed while the device is in contact with the subject, e.g., while affixed to the subject. In some embodiments, the device may be repeatedly applicable to the skin of the subject to deliver to and/or withdraw fluid from the skin and/or beneath the skin of a subject, e.g., at the same location, or at different locations on the skin of the subject. In some aspects, the devices may be self-contained and/or have a relatively small size, and in some cases, the device may be sized such that it is wearable and/or able to be carried by a subject. For example, the device may have a mass and/or dimensions that allow the device to be carried or worn by a subject for various periods of time, e.g., at least about an hour, at least about a day, at least about a week, etc., or no more than about an hour, no more than about 10 min, etc.