Radial artery spasm (RAS) represents a common complication of transradial cardiac catheterization that can occur at any time during the procedure. As a vessel in the distal extremity, the radial artery has a relatively high propensity to spasm in response to mechanical stimulation and circulating humoral factors, such as catecholamines. In the absence of pharmacological prophylaxis, the reported incidence of RAS during transradial cardiac catheterization is between 20.4% and 34%. However, in contemporary practice, rates tend to be lower, mostly ranging from 1% to 23%, though higher rates have also been reported. The wide variation in the reported incidence of RAS is due to both the lack of a standard definition for RAS and differences in preventative measures employed by different studies. RAS is significant not only because it creates discomfort for patients, but also because it jeopardizes the successful completion of the procedure. This chapter will focus on pharmacological prevention of RAS. Diagnosis and treatment of RAS will be discussed in more detail in Chapter 26.