How do I concatenate two strings in Abap/4?

For all SAP Versions
STR_LENGTH = STRLEN( STRING1 ).
MOVE STRING1 TO STRING3.
WRITE STRING2 TO STRING3+STR_LENGTH.
For SAP Version 3.0 choose:

CONCATENATE STRING1 STRING2 INTO STRING3.

If you want a space between both fields:

CONCATENATE STRING1 STRING2 INTO STRING3 SEPARATED BY ' '.

For SAP Version 2.2 choose Functions:

STRING_CONCATENATE for 2 Strings and
STRING_CONCATENATE_3 for 3 Strings.