A dental composition comprising (a) one or more compounds having at least one polymerizable double bond; (b) a polymerization initiator system comprising a compound of the following formula: X-R wherein X is a group of the following formula: wherein M is Si or Ge; R 1 represents a substituted or unsubstituted hydrocarbyl or hydrocarbylcarbonyl group; R 2 represents a substituted or unsubstituted hydrocarbyl or hydrocarbylcarbonyl group; R 3 represents a substituted or unsubstituted hydrocarbyl group; and R (i) has the same meaning as X, whereby the compound of formula may be symmetrical or unsymmetrical; or (ii) is a group of the following formula: wherein Y represents a single bond, an oxygen atom or a group NR', wherein R' represents a substituted or unsubstituted hydrocarbyl group; R 4 represents a substituted or unsubstituted hydrocarbyl group, a trihydrocarbylsilyl group, a mono(hydrocarbylcarbonyl)dihydrocarbylsilyl group or a di(hydrocarbylcarbonyl)monohydrocarbylsilyl group; or (iii) when M is Si, R may be a substituted or unsubstituted hydrocarbyl group. (a) one or more compounds having at least one polymerizable double bond; (b) a polymerization initiator system comprising a compound of the following formula: X-R wherein X is a group of the following formula: wherein M is Si or Ge; R 1 represents a substituted or unsubstituted hydrocarbyl or hydrocarbylcarbonyl group; R 2 represents a substituted or unsubstituted hydrocarbyl or hydrocarbylcarbonyl group; R 3 represents a substituted or unsubstituted hydrocarbyl group; and R (i) has the same meaning as X, whereby the compound of formula may be symmetrical or unsymmetrical; or (ii) is a group of the following formula: wherein Y represents a single bond, an oxygen atom or a group NR', wherein R' represents a substituted or unsubstituted hydrocarbyl group; R 4 represents a substituted or unsubstituted hydrocarbyl group, a trihydrocarbylsilyl group, a mono(hydrocarbylcarbonyl)dihydrocarbylsilyl group or a di(hydrocarbylcarbonyl)monohydrocarbylsilyl group; or (iii) when M is Si, R may be a substituted or unsubstituted hydrocarbyl group. X is a group of the following formula: wherein M is Si or Ge; R 1 represents a substituted or unsubstituted hydrocarbyl or hydrocarbylcarbonyl group; R 2 represents a substituted or unsubstituted hydrocarbyl or hydrocarbylcarbonyl group; R 3 represents a substituted or unsubstituted hydrocarbyl group; and M is Si or Ge; R 1 represents a substituted or unsubstituted hydrocarbyl or hydrocarbylcarbonyl group; R 2 represents a substituted or unsubstituted hydrocarbyl or hydrocarbylcarbonyl group; R 3 represents a substituted or unsubstituted hydrocarbyl group; and R (i) has the same meaning as X, whereby the compound of formula may be symmetrical or unsymmetrical; or (ii) is a group of the following formula: wherein Y represents a single bond, an oxygen atom or a group NR', wherein R' represents a substituted or unsubstituted hydrocarbyl group; R 4 represents a substituted or unsubstituted hydrocarbyl group, a trihydrocarbylsilyl group, a mono(hydrocarbylcarbonyl)dihydrocarbylsilyl group or a di(hydrocarbylcarbonyl)monohydrocarbylsilyl group; or (iii) when M is Si, R may be a substituted or unsubstituted hydrocarbyl group. (i) has the same meaning as X, whereby the compound of formula may be symmetrical or unsymmetrical; or (ii) is a group of the following formula: wherein Y represents a single bond, an oxygen atom or a group NR', wherein R' represents a substituted or unsubstituted hydrocarbyl group; R 4 represents a substituted or unsubstituted hydrocarbyl group, a trihydrocarbylsilyl group, a mono(hydrocarbylcarbonyl)dihydrocarbylsilyl group or a di(hydrocarbylcarbonyl)monohydrocarbylsilyl group; or Y represents a single bond, an oxygen atom or a group NR', wherein R' represents a substituted or unsubstituted hydrocarbyl group; R 4 represents a substituted or unsubstituted hydrocarbyl group, a trihydrocarbylsilyl group, a mono(hydrocarbylcarbonyl)dihydrocarbylsilyl group or a di(hydrocarbylcarbonyl)monohydrocarbylsilyl group; or (iii) when M is Si, R may be a substituted or unsubstituted hydrocarbyl group.