In the Prolegomena to the edition of al-Khwarizmi's 'Algebra', van Roomen says that his intention is to examine what kind of science the algebra or 'ars analyticais', by looking at its origin and history. First of all he gives a summary of the different names which have been proposed for this science. Then he examines the place of the 'ars analytica' among the other sciences. It deals with quantities, their equality or inequality, their proportion and proportionality. For this reason it belongs to mathematics. Those who have written about algebra considered it to be a part of arithmetic, although it might as well be considered to be part of geometry. Algebraic propositions usually are demonstrated by geometrical constructions, so that algebra should perhaps better be considered as a part of geometry. Van Roomen ... prefers to classify the algebra or analytical science as belonging to the 'Mathesin prima', which deals with quantity in general. The material object of the 'ars analytica' or algebra is indeed quantity. The formal object, however, is the equality (aequalitas) of quantities, since only those problems, in which some equation is either explicitly given or can be deduced from the data of the problem, are analytic.One of van Roomen's most impressive results was finding π to 16 decimal places. He did this in 1593 using 230 sided polygons. Van Roomen's interest in π was almost certainly as a result of his friendship with Ludolph van Ceulen. Van Roomen worked on trigonometry and the calculation of chords in a circle. In 1596 Rheticus's trigonometric tables Opus palatinum de triangulis were published, many years after Rheticus died. Van Roomen was critical of the accuracy of these tables and wrote to Clavius at the Collegio Romano in Rome pointing out that, to calculate tangent and secant tables correctly to ten decimal places, it was necessary to work to 20 decimal places for small values of sine, see [3]. In 1600 van Roomen visited Prague where he met Johannes Kepler and told him of his worries about the methods employed in Rheticus's trigonometric tables.Among other contributions made by van Roomen was one to figures of equal perimeter. Pappus had proved a number of results concerning the maximum area of polygons of equal perimeter. For example regular n-sided polygons have the maximum area among all n-sided polygons of fixed perimeter. Van Roomen generalised the results of Pappus and, again showing his precise thinking, realised that 'regular' had not been properly defined. His work in this area is discussed in detail in [9].