The tongue is an essential component of the upper airway and is the primary target of myofunctional therapy (MFT) for the management of obstructive sleep apnea (OSA). As such, standardized, reliable, and valid assessment of tongue function is a critical component of OSA assessment. This chapter provides descriptions and practical information about assessments for tongue resting posture, ankyloglossia, and tongue function, including muscle strength, endurance, and tone based on current literature. Techniques involve low-tech approaches such as rating scales and simple linear measurements, and instrumental approaches involving electromyography and lingual manometry. The approaches selected by the orofacial myologist will depend upon such factors as cost, availability, training, ease of use, clinical needs, and clinical population. There are multiple valid and clinically feasible methods for the assessment of ankyloglossia and tongue strength. Assessment of tongue muscle tone has proved to be the most elusive, as no available techniques have yet been validated for this purpose. Instead, inferences about lingual tone are derived from measures of tongue strength and from evidence of structural collapse during episodes of apnea or hypopnea. Measures of tongue function can be compared to standard measures of OSA, specifically the Apnea-Hypopnea Index, to determine if improving tongue muscle function through MFT corresponds to reduced OSA severity.