Protein tyrosine phosphatases (PTPs) are key regulatory enzymes in signal transduction pathways and their aberrancy has been implicated in various diseases such as cancers, metabolic syndrome, and autoimmune disorders. In spite of its great importance, determination of the functional significance of PTPs remains a major challenge, and efficient methodologies are needed to specifically delineate PTP functions. Besides the strategy to use potent and selective PTP inhibitors to study the physiological roles of the enzymes, measurement of PTP activities using specific PTP substrates or activity-based probes (ABPs) has been reported. This review focused on the recent development of small molecular probes to detect PTP activities, consisting of five sub-categories: 1. Conventional fluorescent substrates; 2. Ratiometric fluorescent PTP substrates; 3. Fluorescence substrates with selectivity to a single PTP or a class of PTPs; 4. ABPs specific for PTPs; and 5. A real-time imaging of PTP-substrate complex using a small molecule PTP probe which, offers a measurement of a real-time activity of a certain PTP in cells.