A process valve with a housing aggregation for monitoring, open-loop and closed-loop control systems for a process valve, which has at least one valve stem bearing a closing member and a drive supplied with pressure medium for the valve stem(s), with the housing aggregation (I, II, III, IV) of modular structure, • which on the side of the drive remote from the process valve is detachably connected thereto and into which a drive rod connected to the valve stem(s) is guided, • which has a position indicator which is contacted with the drive rod and which ascertains the current position of the valve stem in relation to a fixed reference system and delivers current position indications in relation to this, • which has means for storing the current position indications and means for calculating and for comparing position indications, and • which has means for optical and/or acoustic notification and/or for reading out the function state of the process valve, characterised in that • the respective housing aggregation (I, II, III, IV) is fastened via a rotationally symmetrical cover part to the respective drive in positive and non-positive manner, • the housing aggregation (I, II, III, IV) furthermore can be assembled as desired from a first housing top attachment, a second housing top attachment of first, second or third type (3, 3*, 3**) and a first or second hood (4, 4.1), • the housing top attachment (2, 3, 3*, 3**) and the hood (4, 4.1) are formed in each case as a rotationally symmetrical basic body with a cylindrical outer contour, • connection points (6/2, 2/4; 6/3, 3/4; 6/3, 3/3*, 3*/4; 6/3, 3/3**, 3**/4.1) between the cover part, housing top attachment (2, 3, 3*, 3**) and hood (4, 4.1) of the respective housing aggregation (I, II, III, IV) are formed, on which points, viewed in meridian section, the envelope lines of the cover part, of the basic bodies of the housing top attachments (2, 3, 3*, 3**) and of the hood (4, 4.1) are arranged in a row, with identical diameters, in flush manner, contiguously and without gaps, • all the connection points are sealed off from each other, • and in that the respective connection partners are centred identically relative to each other and are connected to one another in positive and non-positive manner at all the connection points. • which on the side of the drive remote from the process valve is detachably connected thereto and into which a drive rod connected to the valve stem(s) is guided, • which has a position indicator which is contacted with the drive rod and which ascertains the current position of the valve stem in relation to a fixed reference system and delivers current position indications in relation to this, • which has means for storing the current position indications and means for calculating and for comparing position indications, and • which has means for optical and/or acoustic notification and/or for reading out the function state of the process valve, • the respective housing aggregation (I, II, III, IV) is fastened via a rotationally symmetrical cover part to the respective drive in positive and non-positive manner, • the housing aggregation (I, II, III, IV) furthermore can be assembled as desired from a first housing top attachment, a second housing top attachment of first, second or third type (3, 3*, 3**) and a first or second hood (4, 4.1), • the housing top attachment (2, 3, 3*, 3**) and the hood (4, 4.1) are formed in each case as a rotationally symmetrical basic body with a cylindrical outer contour, • connection points (6/2, 2/4; 6/3, 3/4; 6/3, 3/3*, 3*/4; 6/3, 3/3**, 3**/4.1) between the cover part, housing top attachment (2, 3, 3*, 3**) and hood (4, 4.1) of the respective housing aggregation (I, II, III, IV) are formed, on which points, viewed in meridian section, the envelope lines of the cover part, of the basic bodies of the housing top attachments (2, 3, 3*, 3**) and of the hood (4, 4.1) are arranged in a row, with identical diameters, in flush manner, contiguously and without gaps, • all the connection points are sealed off from each other, • and in that the respective connection partners are centred identically relative to each other and are connected to one another in positive and non-positive manner at all the connection points.