Models of decision making and response time (RT) are often formulated using stochastic differential equations (SDEs). Researchers often investigate these models using a simple Monte Carlo method based on Euler's method for solving ordinary differential equations. The accuracy of Euler's method is investigated and compared to the performance of more complex simulation methods. The more complex methods for solving SDEs yielded no improvement in accuracy over the Euler method. However, the matrix method proposed by Diederich and Busemeyer (2003) yielded significant improvements. The accuracy of all methods depended critically on the size of the approximating time step. The large (∼10 ms) step sizes often used by psychological researchers resulted in large and systematic errors in evaluating RT distributions.Over the past 40 years, models of response time (RT) for simple decision making have become very successful at capturing the details of observed data (Audley & Pike, 1965;Brown & Heathcote, 2005;Busemeyer & Townsend, 1993;Diederich, 1997;Heath, 1981;LaBerge, 1962;Lacouture and Marley, 1991;Laming, 1966;Link & Heath, 1975;Ratcliff, 1978; Ratcliff & Smith, 2004, Appendix;Ratcliff, Van Zandt, & McKoon, 1999;Smith, 1995;Vickers, 1970;Vickers & Lee, 2000). More recently, the same models have also become quite successful at explaining decision making at a neural level (Carpenter & Reddi, 2001;Cook & Maunsell, 2002;Glimcher 2003;Gold & Shadlen, 2001;Ratcliff, Cherian & Seagraves, 2003;Reddi & Carpenter, 2000;Roitman & Shadlen, 2002;Sato, Murthy, Thompson & Schall, 2001;Sato & Schall, 2003;Shadlen, Britten, Newsome & Movshon, 1996;Wang, 2002). The most successful models of decision making in both cognitive and neural domains are the sequential sampling models. These models are based on the idea that noisy stimulus information is accumulated progressively over time until sufficient information for one of the response alternatives has been obtained. The predicted decision time in such models is obtained mathematically by solving a first-passage-time (FPT) problem, that is, the time taken for the accumulated information to reach a criterion and trigger a response. For some models, there exist explicit analytic methods or highly accurate numerical methods for solving the FPT problem (see Ratcliff & Smith, Appendix, for a survey). For other models, this problem may be complex or intractable. In such situations, researchers must resort to Monte Carlo simulation techniques to obtain predicted RT distributions and choice probabilities. We investigate the properties of such simulation techniques in this article.One of the best-known sequential sampling models, and one that has been applied to a wide range of experimental data, is the diffusion model of Ratcliff (1978;Ratcliff & Rouder, 1998). This model assumes that evidence accumulation begins at some initial value (z) and